GIFT  or 


PHYSICS   OF   THE    HOUSEHOLD 


THE  MACMILLAN  COMPANY 

NEW  YORK  •    BOSTON  •    CHICAGO  •    DALLAS 
ATLANTA   •    SAN   FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •    BOMBAY  •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


PHYSICS 


OF    THE 


HOUSEHOLD 


BY 


CARLETON   JOHN    LYNDE,    PH.D. 

PROFESSOR   OF    PHYSICS    IN    MACDONALD 
COLLEGE,   CANADA 


gorfe 
THE    MACMILLAN   COMPANY 

1917 

All  rights  reserved 


COPYRIGHT,  1914, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.  Published  May,  1914.  Reprinted 
August,  October,  1914;  May,  October,  1915  ;  July,  December, 
1916  ;  July,  December,  1917. 


NortnonU 

J.  8.  Cashing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

THIS  is  an  elementary  textbook  of  physics,  written  for  stu- 
dents of  household  science.  It  covers  the  ground  usually  cov- 
ered by  elementary  textbooks,  but  differs  from  many  of  them  in 
two  ways :  first,  the  illustrative  examples  and  applications  are 
taken  largely  from  the  home;  second,  the  common  system  of 
weights  and  measures  is  used,  in  addition  to  the  metric  system. 

The  writer  believes  that  we  teach  physics  to  young  students 
for  these  reasons :  first,  that  they  may  obtain  knowledge  of  the 
physical  world  about  them ;  and  second,  that  they  may  gain, 
through  this  knowledge,  the  power  to  control  the  forces  of  nature 
for  their  own  benefit,  and  for  the  benefit  of  others.  In  other 
words,  we  wish  them  to  acquire  knowledge  which  they  will  use 
in  everyday  life. 

The  reason  for  using  illustrations  taken  from  household  appli- 
ances known  to  the  student  is  obvious.  It  is  good  pedagogy 
to  lead  from  the  known  to  the  unknown,  and  to  illustrate  the 
unknown  by  means  of  the  known.  This  is  the  method  followed 
in  this  book. 

The  use  of  the  common  system  of  weights  and  measures  is, 
in  the  writer's  opinion,  justified  by  the  desirability  of  having 
the  students  acquire  their  knowledge  in  terms  of  the  weights 
and  measures  which  they  must  use  should  they  ever  apply  the 
knowledge  in  everyday  life.  Furthermore,  the  writer,  although 
a  strong  advocate  of  the  metric  system,  believes  that  it  is  peda- 
gogically  unsound  to  try  to  teach  elementary  physics  by  means 
of  the  metric  system  exclusively.  It  is  an  attempt  to  teach  an 
unknown  subject  by  means  of  an  unknown  system  of  weights 
and  measures  and  leads  to  confusion  and  lack  of  power  on  the 


459853 


Vi  PREFACE 

part  of  the  student.  Long  experience  leads  the  writer  to  believe 
that  the  correct  method  is  to  introduce  the  subject  by  means  of 
the  common  system,  later  to  teach  the  metric  system  and  point 
out  its  advantages,  and  then  to  use  the  two  systems  side  by  side. 
This  is  the  method  followed  in  this  book. 

The  writer  wishes  to  thank  the  Macmillan  Company  for  per- 
mission to  use  illustrations  from  other  textbooks  published  by 
them.  He  has  used  illustrations  from:  "  An  Elementary  Course 
of  Physics  "  by  Aldous  ;  "  A  Classbook  of  Physics  "  by  Gregory 
and  Hadley ;  "  Elementary  General  Science  "  by  Simmons  and 
Jones ;  "  Elements  of  Physics  "  by  Andrews  and  Rowland ; 
"  Lessons  in  Elementary  Physics  "  by  Balfour  Stewart ;  "  Sci- 
ence of  Common  Life"  by  Simmons  and  Stenhouse ;  "College 
Physics"  by  Reed  and  Guthe ;  "  A.  Textbook  of  Physics  "  by 
Spinney ;  "  Elementary  Lessons  in  Electricity  and  Magnetism  " 
by  Sylvanus  P.  Thompson ;  "  Heat,  Light,  and  Sound "  by 
D.  E.  Jones ;  "  Elements  of  Physics  "  by  Crew  and  Jones.  In 
addition,  the  writer  wishes  to  thank  the  Stover  Mfg.  Co.,  of 
Freeport,  111.,  for  figure  13,  the  Andrews  Heating  Co.,  of  Minne- 
apolis, Minn.,  for  figure  45,  Messrs.  Fay  and  Bowen  of  Geneva, 
N.Y.,  for  figure  158,  and  Messrs.  Sturgis  &  Walton,  of  New 
York,  for  permission  to  use  illustrations  from  the  writer's  book, 
"  Home  Waterworks." 

In  conclusion  the  writer  wishes  to  thank  Dr.  H.  C.  Sherman, 
of  Columbia  University,  for  many  valuable  suggestions,  and 
Mr.  A.  Norman  Shaw,  of  Macdonald  College,  for  assistance  with 
the  proofreading  and  for  excellent  suggestions. 

C.  J.  L. 


CONTENTS 

CHAPTER   I 

PAGE 

MECHANICS  —  SOLIDS 1 

Mechanical  appliances  in  the  home.  Lever  appliances. 
Mastery.  Wheel  and  axle  appliances. 

CHAPTER   II 

MECHANICAL  APPLIANCES  (continued} 13 

Pulley  appliances.     Screw  appliances. 

CHAPTER   III 
WORK 20 

Units  of  work.     The  law  of  work. 

i 

CHAPTER   IV 
MEASUREMENT 24 

The  common  system.  The  metric  system.  Advantages  of 
the  metric  system. 

CHAPTER  V 

MECHANICS  —  LIQUIDS 30 

Water  supply.  Laws  relating  to  pressure  in  liquids.  Pascal's 
law.  The  law  of  Archimedes.  Density. 

CHAPTER  VI 
MECHANICS  —  GASES 49 

Air  has  weight  Atmospheric  pressure.  The  barometer. 
Laws  which  apply  to  gases.  Boyle's  law.  Henry's  law. 

CHAPTER  VII 

AIR  APPLIANCES .       .       .61 

Pumps.  Pneumatic  tank  water  supply  system.  The  hydraulic 
ram.  Vacuum  cleaners.  The  fire  extinguisher.  Traps.  The 
gas  meter.  Appliances  which  use  compressed  air. 

vii 


viii  CONTENTS 

CHAPTER  VIII 

PAGE 

HEAT  IN  THE  HOME 79 

The  fire  the  center  of  the  home.  Expansion.  Thermometers. 
Expansion  of  gases  in  cooking.  Why  a  stove  draws.  Draft  in 
a  kitchen  range.  Hot-air  heating  system.  Hot-water  heating 
system.  The  hot-water  tank.  Nature  of  heat. 

CHAPTER   IX 

MOVEMENT  OF  HEAT.     HEAT  APPLIANCES 102 

Conduction,  convection,  and  radiation.  Cooking  utensils. 
Fireless  cooker.  Thermos  bottle.  Walls  of  houses.  Clothes. 
Ventilation. 

CHAPTER   X 

MEASUREMENT  OF  HEAT 116 

The  British  Thermal  Unit  and  calorie.  Comparing  fires, 
cooking  utensils,  fireless  cookers,  and  foot  warmers.  Cooling 
effect  of  ice.  Heating  effect  of  steam. 

CHAPTER  XI 

HEAT  CAPACITY,  SPECIFIC  HEAT,  LATENT  HEAT  ....    126 

Applications  of  latent  heat.  Refrigerator.  Freezing  mix- 
tures. Artificial  ice  machine.  Steam  heating.  Steam  cookers. 
Distillation. 

CHAPTER  XII 

EVAPORATION,  DEW  POINT,  BOILING  POINT 144 

Why  clothes  dry.  Causes  of  cloud,  rain,  hail,  snow,  and  dew. 
Change  in  boiling  point.  Applications  of  boiling. 

CHAPTER  XIII 

SOURCES  OF  HEAT.    HEAT  AND  WORK 152 

Cost  of  fuels.  Steam  engine.  Gasoline  engine.  Horse 
power. 

CHAPTER  XIV 

ELECTRICITY  IN  THE  HOME 161 

Household  electrical  appliances.     Electric  cells. 


CONTENTS  ix 

CHAPTER  XV 

PAGE 

MAGNETISM  AND  THE  ELECTROMAGNET 169 

Magnetic  induction.  Magnetic  field.  How  magnets  are  made. 
The  electromagnet  The  electric  bell.  The  electric  telegraph. 

CHAPTER   XVI 

THE  ELECTRIC  MOTOR  IN  THE  HOME 182 

How  the  electric  client  runs  the  motor.  The  motor  in  the 
kitchen. 

CHAPTER   XVII 

ELECTRIC  HEATING,  COOKING,  AND  LIGHTING  APPLIANCES  IN  THE 

HOME 187 

Electric  iron,  stove,  coffee  percolator,  oven,  etc.  The  incan- 
descent lamp.  The  arc  lamp. 

CHAPTER   XVIII 

ELECTROPLATING 195 

Electrolysis.  Electroplating.  Laws  of  electrolysis.  The 
storage  cell. 

CHAPTER  XIX 

ELECTRICAL  TERMS  AND  MEASURES 202 

The  ampere,  ohm,  volt,  and  watt.  Ohm's  law.  Joule's  law. 
Electrical  terms  applied  to  common  appliances.  Electrical  cal- 
culations. 

CHAPTER  XX 

MEASURING  INSTRUMENTS,  SERIES  AND  PARALLEL  CONNECTIONS  .    212 
Galvanometer.     Voltmeter.     Ammeter.     Resistance.  "yLO 

CHAPTER  XXI 

INDUCED  CURRENTS.    THE  DYNAMO 219 

How  induced  currents  are  produced.      The  direction  of  in- 
duced currents.      Alternating-current  dynamo.     Direct-current  ' 
dynamo.     Sources  of  electrical  energy. 


X  CONTENTS 

CHAPTER  XXII 

PAGE 

INDUCED  CURRENTS  (continued} 230 

Transformer.     Induction  coil.     Telephone.    (/—^> 

V 

CHAPTER   XXIII 

WIRELESS  TELEGRAPH.     CATHODE  RAYS.    X  RAYS.     RADIUM        .    237 

Wireless  sending  and  receiving  stations.  Properties  of 
cathode  rays.  Nature  of  X  rays.  Radium. 

CHAPTER   XXIV 

LIGHT  IN  THE  HOME.- 246 

Arrangement  of  lighting  fixtures  in  the  home.  Intensity  of 
illumination.  Nature  of  light. 

CHAPTER  XXV 

REFLECTION  AND  REFRACTION  OF  LIGHT 253 

Law  of  reflection.     Laws  of  refraction. 

CHAPTER   XXVI 

LENSES  AND  OPTICAL  INSTRUMENTS 259 

Real  and  virtual  images.  Camera.  Projecting  lantern.  Eye. 
Spectacles.  Magnifying  glass.  Telescope.  Compound  micro- 
scope. Opera  glass.  Stereoscope. 

CHAPTER   XXVII 

COLOR 269 

Composite  nature  of  white  light.  Why  objects  are  colored. 
The  Young-Helmholtz  theory  of  color  vision. 

CHAPTER  XXVIII 
SOUND 274 

How  sound  is  produced.  Nature  of  sound  waves.  Noises 
and  musical  sounds.  Pitch. 


CONTENTS  Xi 

CHAPTER   XXIX 

PAGE 

Music  AND  MUSICAL  INSTRUMENTS 279 

Musical  scale.  Stringed  instruments.  Fundamental,  octaves, 
and  overtones.  Quality.  Wind  instruments.  Resonance. 
Harmony  and  discord.  The  phonograph. 

CHAPTER  XXX 

A  FURTHER  STUDY  OF  MECHANICS 289 

Gravitation.  Composition  of  forces.  Falling  bodies.  New- 
ton's laws  of  motion.  Units  of  forco^Units  of  work.  Poten- 
tial and  kinetic  energy. 

APPENDIX 

WEIGHTS  AND  MEASURES 303 

THE  METRIC  SYSTEM 305 

EQUIVALENTS       .  ,  307 


PHYSICS  OF  THE  HOUSEHOLD 


CHAPTER  I 
MECHANICS.     SOLIDS 

MECHANICAL  APPLIANCES  IN  THE  HOME 

IN  the  first  two  chapters  we  shall  study  some  of  the  household 
mechanical  appliances  which  are  related  to  the  lever,  wheel  and 
axle,  screw,  and  pulley. 

LEVERS 


A  simple  lever  is 
is  suspended  from  a 
string  and  balanced, 
then  a  weight  of  2  lb., 
8  in.  from  the  turning 
point,  is  balanced  by 
a  weight  of  i  lb.,  16  in. 
from  the  turning  point 
on  the  other  side. 

It  will  be  noticed 
that  when  this  lever  is 
balanced,  the  weight 
on  one  side,  multiplied 
by  its  distance  from 
the  turning  point,  is 
equal  to  the  weight  on 
the  other  side,  multi- 
plied by  its  distance 
from  the  turning  point ; 
that  is,  2  X  8  =  i  X  16. 
This  is  the  law  of  the 


represented  in  Fig.  i.      A  yardstick 


...... 


FIG.  i.  —  A  simple  lever. 


°F  THE  HOUSEHOLD 


lever  and  is  true  in  all  cases;  for  example,  2  lb./5  in.  from 
the  turning  point,  is  balanced  by  i  lb.,  10  in.  from  the  turning 
point,  and  2  X  5  =  i  X  10 ;  also  3  lb.,  at  4  in.,  is  balanced 
by  i  lb.,  at  12  in.,  and  3X4=1X12;  etc. 

Definitions.  —  The  turning  point  of  any  lever  is  called  the 
fulcrum  (see  Fig.  2).     The  product  obtained  by  multiplying  a 


Force 


Pum-p  Handle. 


T&ck  Lifter 


Fulcrum. 


FIG.  2.  —  Household  lever  appliances  showing  position  of  fulcrum, 
force,  and  weight. 

weight  by  its  distance  from  the  fulcrum  is  called  the  moment  of 
the  weight.  The  weight  or  force  applied  to  any  lever,  by  the 
hand  or  otherwise,  is  called  simply  the  force.  The  distance  from 
the  force  to  the  fulcrum  is  called  the  force  arm.  The  weight, 
pressure,  or  lift  exerted  by  a  lever  is  called  simply  the  weight. 


MECHANICS.     SOLIDS  3 

The  distance  from  the  weight  to  the  fulcrum  is  called  the  weight 
arm. 

The  lever  law.  —  Any  lever  is  balanced  when  the  moment  on 
one  side  of  the  fulcrum  is  equal  to  the  moment  on  the  other. 
This  is  the  lever  law.  It  can  be  stated  also  as  follows:  A  lever 
is  balanced  when,  weight  X  weight  arm  =  force  X  force  arm. 

If  there  are  a  number  of  weights  on  each  side  of  the  fulcrum, 
the  lever  is  balanced  when  the  sum  of  the  moments  on  one  side 
is  equal  to  the  sum  of  the  moments  on  the  other. 

Lever  appliances.  —  A  number  of  lever  appliances  are  shown 
in  Fig.  2,  namely,  the  scissors,  pump  handle,  tack  lifter,  and 
pliers.  Since  these  appliances  are  levers,  the  lever  law  holds 
for  them.  The  lever  law  is,  when  a  lever  is  balanced,  weight 
X  weight  arm  =  force  X  force  arm.  There  are  four  quantities 
in  this  equation,  and  if  we  know  any  three  of  them,  we  can  cal- 
culate the  fourth. 

In  the  case  of  the  tack  lifter  shown  above,  it  has  been 
found  by  measurement  that  the  weight  arm  is  i  in.  long  and 
the  force  arm  12  in.  long.  Let  us  calculate  the  weight  or 
lift  produced  when  we  exert  10  Ib.  of  force  on  the  handle. 
We  do  this  as  follows: 

Weight  X  weight  arm  =  force  X  force  arm 
Weight  X  i  =  10  X  12 
Weight  =  i2olb. 

That  is,  if  we  apply  10  Ib.  of  force  on  the  handle,  the  lift  given 
to  the  tack  is  120  Ib. 

In  the  case  of  the  pump  handle  it  has  been  found  by  measure- 
ment that  the  weight  arm  is  4  in.,  and  the  force  arm  20  in.  long. 
Let  us  calculate  the  weight  or  lift  produced  when  we  apply 
25  ib.  of  force  to  the  handle.  As  before  : 

Weight  X  weight  arm  =  force  X  force  arm 
Weight  X  4  =  25  X  20 
Weight  =  i25lb. 


4  PHYSICS  OF  THE  HOUSEHOLD 

That  is,  if  we  apply  25  Ib.  force  to  the  handle,  we  produce  a  lift 
of  125  Ib.  on  the  pump  rod. 

In  the  case  of  the  pliers  it  has  been  found  by  measure- 
ment that  the  weight  arm  is  f  in.  long  and  the  force  arm  6  in. 
long.  Let  us  calculate  the  force  necessary  to  produce  40  Ib. 
weight  or  pressure. 

Weight  X  weight  arm  =  force  X  force  arm 
40  X  f  =  force  X  6 
30  =  force  X  6 
5  =  force 

That  is,  an  object  in  the  jaws  of  the  pliers  is  subjected  to  40  Ib. 
pressure  when  we  exert  5  Ib.  force  in  drawing  the  handles 
together. 

By  a  similar  calculation  we  find  that  in  the  case  of  the  scissors 
shown  above  each  pound  of  force  used  to  pull  the  handles  to- 
gether produces  3  Ib.  pressure  on  the  object  between  the  blades. 

Mastery.  —  These  examples  show  how  the  lever  law  gives  us 
greater  mastery  over  lever  appliances.  We  have  greater  mas- 
tery because  we  know  why  and  how  much  the  appliance  helps 
us,  also  we  know  how  the  appliance  can  be  altered  to  help  us 
still  more. 

Let  us  consider  these  points  for  the  case  of  the  tack  lifter. 
The  reason  why  the  tack  lifter  helps  us  is  that  it  acts  as  a  lever 
and  changes  the  small  force  applied  by  the  hand  to  a  large  lift 
exerted  on  the  tack. 

We  find  out  how  much  it  helps  us  by  dividing  the  force  arm 
by  the  weight  arm.  This  gives  us  the  advantage  of  the  lever 
or  how  much  it  helps  us.  In  this  case  it  is  --£-  =  12.  That  is, 
each  pound  of  force  exerted  by  the  hand  is  multiplied  by  12,  and 
produces  12  Ib.  lift  on  the  tack. 

We  can  see  that  there  are  two  ways  in  which  the  tack  lifter 
could  be  altered  to  help  us  still  more ;  namely,  by  lengthening 
the  force  arm  or  by  shortening  the  weight  arm.  For  example, 
if  the  force  arm  were  made  twice  as  long,  or  24  in.,  the  ad- 


MECHANICS.     SOLIDS  5 

vantage  of  the  lever  would  be  %f-  =  24,  or  twice  as  great ;  that 
is,  each  pound  of  force  would  be  turned  into  a  lift  of  24  Ib. 
instead  of  12  Ib.  Similarly,  if  the  weight  arm  were  made  one 
half  as  long,  or  f  in.,  the  force  arm  being  the  same,  12  in.,  the 

advantage  would  be  twice  as  great,  -^   =  24.     That  is,  each 

2 

pound  of  force  would  produce  a  lift  of  24  Ib.  instead  of  12  Ib. 

Three  classes  of  levers.  K  T 

T  j-  -j  j  •  <.  fulcrum 

—  Levers  are  divided  into 

three  classes  according   to  £•.»-* ---->y<- > 

the  relative  positions  of  the  J— .      ^ 

weight,  fulcrum,  and  force.  Weight                                 Force 

Levers  in  which  the  ful-  (1)  Lever  of  1st  Class. 
Crum  is  between  the  weight 

and  the  force  are  known  as  &&&**          FA               force 

\eversoithefirstclass.     See  ^~ '*' J 

(i),  Fig.  3.    The  appliances 
which  we  studied  above  are 

levers  of  this  class. 

(2)  Lever  of  2nd  Class. 
Levers     in     which     the 

weight  is  between  the  ful-  .Force 

crum    and    the    force    are  FA 

levers  of   the  second  class,         &• _  --j 

and  those  in  which  the  force          j  WA  y  ' 

is  applied  at  some  point  be-    *  "fCTum  Weight 

tween  the  fulcrum  and  the         ™  =force  arm.    WA-weightarm. 

(3)  Lever  of  3rd  Class, 
weight    are    levers   of    the       ... 

FIG.  3.  — The  three  classes  of  levers. 

third  class. 

Levers  of  the  second  class.  —  The  appliances  shown  in  Fig.  4 
are  levers  of  the  second  class,  because  the  weight  is  between  the 
fulcrum  and  the  force. 

The  lever  law  applies  to  these  levers,  and  if  we  find  the 
weight  arm  and  force  arm  of  each  by  measurement,  we  can 
calculate  the  relation  between  the  force  and  weight  in  each 
case. 


PHYSICS  OF  THE  HOUSEHOLD 


In  the  case  of  the  can  opener,  the  weight  arm  is  i  in.  long, 
and  the  force  arm  6  in.  long;  therefore,  i  Ib.  of  force  pro- 
duces a  cutting  pressure  of  6  Ib.,  5  Ib.  of  force  a  cutting  pressure 
of  30  Ib.,  etc.  Similarly,  i  Ib.  of  force  exerted  in  drawing  to- 
gether the  handles  of  the  nut  cracker,  lemon  squeezer,  or  fruit 


Force 


ftl/CJt 


Can  Op 


fulcrum 


Fulcrum 


or  Pressure 

^Squeezer  fruit  Press. 

FIG.  4.  —  Levers  of  the  second  class. 


press  produces  a  weight  or  pressure  of  4  Ib.,  4  Ib.,  or  6  Ib.,  re- 
spectively. 

Levers  of  the  third  class.  —  The  appliances  shown  in 
Fig.  5  are  levers  of  the  third  class,  because  in  each  case 
the  force  is  applied  at  a  point  between  the  fulcrum  and  the 
weight. 

The  lever  law  applies  to  these  levers,  and  by  measuring  the 
force  arm  and  weight  arm  of  each  we  can  calculate  the  relation 
between  the  weight  and  force  as  above. 


MECHANICS.     SOLIDS 


It  will  be  noticed  that  in  levers  of  the  third  class  the  force 
arm  is  shorter  than  the  weight  arm;  therefore,  the  weight  or 
pressure  produced  is  always  less  than  the  force  exerted. 


JFbrce 

Grass  Cutter  Pressure 

FIG.  5.  —  Levers  of  the  third  class. 

WHEEL  AND  AXLE 

The  wheel  and  axle,  Fig.  6,  consists  of  a 
large  and  small  wheel  fastened  together  or 
fastened  to  the  same  axle.  //  is  in  reality  a 
lever;  for  example,  in  Fig.  6,  C  is  the  fulcrum, 
P  is  the  force,  AC,  the  radius  of  the  large 
wheel,  is  the  force  arm,  Q  is  the  weight,  and 
CB,  the  radius  of  the  small  wheel,  is  the 
weight  arm.  If  we  find  the  force  arm  and 
weight  arm  by  measurement,  we  can  use  the 


—  Wheel 
axle. 


8 


PHYSICS  OF  THE  HOUSEHOLD 


lever  law  to  find  the  relation  between  the  force  and  weight 
as  we  did  in  the  case  of  the  levers. 

Example.  —  If  AC  is  12  in.  and  CB  is  6  in.,  a  force  P  of  10  Ib. 
will  support  a  weight  Q  of  20  Ib.,  because  according  to  the  lever 
law  the  wheel  and  axle  balances  when 

Weight  X  weight  arm  =  force  X  force  arm 
Weight  X  6  =  10  X  12 


Weight  = 


10  X  12 


20  Ib. 


FIG.  7. — The  windlass. 


The  windlass,  Fig.  7,  is  one  form  of  wheel  and  axle.  It  con- 
sists of  a  drum  turned  by  a  crank.  The  weight  is  attached 
to  a  rope  wound  on  the  drum  and  the 
force  is  applied  to  the  crank  handle.  The 
radius  of  the  drum  is  the  weight  arm,  and 
the  crank  arm  is  the  force  arm. 

Wheel  and  axle  appliances.  —  The  ap- 
pliances shown  in  Fig.  8  are  wheel  and 
axle  appliances  of  the  windlass  type.  The 
force  arm  of  each  is  the  length  of  the 
crank  arm.  The  weight  arm  in  the  grate 
shaker,  wringer,  and  ice-cream  freezer  is 
the  radius  of  the  grate,  of  the  roll,  and  of  the  can,  respectively. 
In  the  ice-cream  freezer  there  are  cogwheel  gears  at  the  top,  but 
they  are  of  the  same  size,  and  thus  do  not  affect  the  relation 
between  the  force  and  the  weight.  In  the  case  of  the  coffee 
mill,  the  weight  arm  is  approximately  the  distance  from  the  axle 
to  the  middle  of  the  grinding  surface.  In  the  case  of  the  bread 
mixer,  the  weight  arm  is  approximately  half  the  radms  of  the 
mixer. 

If  we  measure  the  force  arm  and  weight  arm  of  any  of  these 
appliances,  we  can  calculate  the  relation  between  the  force  and 
weight  by  means  of  the  lever  law. 

Example  i.  —  The  crank  arm  of  a  wringer  is  9  in.  long,  the 
radius  of  the  roll  is  i  in.  *  If  10  Ib.  force  is  applied  to  the 


MECHANICS.     SOLIDS 
ShaZer  *>« 


Ice  Cream  fzeezer 


FIG.  8. — Household  appliances  of  the  windlass  type. 

handle,  what  is  the  pressure  forcing  the  clothes  between  the 
rolls? 

>  yl          7 

Weight  X  weight  arm  =  force  X  force  arm 

Weight  X  i  =  10  X  9 
Weight  or  pressure  =  90  Ib. 

That  is,  if  10  Ib.  force  is  used  on  the  handle,  the  pressure  forcing 
the  clothes  between  the  rolls  is  90  Ib. 

Example  2. — The  handle  of  a  grate  shaker  is  15  in.  long. 
The  radius  of  the  grate  is  3  in.  The  force  is  10  Ib.  With 
what  pressure  is  the  grate  moved  under  the  ashes? 


10  PHYSICS   OF  THE   HOUSEHOLD 

Weight  X  weight  arm  =  force  X  force  arm 

Weight  X  3  =  10  X  15 
Weight  or  pressure  =  50  Ib.  . 

That  is,  10  Ib.  of  force  applied  to  the  handle  produces  a  pressure 
of  50  Ib.  moving  the  grate  under  the  ashes. 

It  will  be  noticed  that  we  can  increase  the  advantage  of  these 
machines,  as  in  the  case  of  the  levers,  either  by  increasing  the 
length  of  the  force  arm  or  by  decreasing  the  length  of  the  weight 
arm. 

EXERCISES 

1.  State  the  law  of  the  lever. 

2.  Define  fulcrum,  moment,  force  arm,  weight  arm. 

3.  A  yardstick  is  balanced  at  the  middle.     If  i  Ib.  is  attached  10  in. 
from  the  fulcrum,  where  must  2  Ib.  be  placed  to  balance  it?     Where 
5  Ib.  ?     Make  a  sketch  of  each. 

4.  If  on  the  yardstick  of  (3)  2  Ib.  is  placed  15  in.  from  the  fulcrum, 
where  will  3  Ib.  balance  it?     Where  5  Ib. ?     Make  a  sketch  of  each. 

5.  If  2  Ib.  is  placed  10  in.  from  the  fulcrum  and  i  Ib.  5  in.  from  the 
fulcrum  on  the  same  side,  where  will  5  Ib.  balance  them?     Make  a 
sketch. 

6.  Describe  how  the  advantage  of  a  lever  is  found. 

7.  The  force  arm  of  a  pump  handle  is  2\  ft.  long  and  the  weight  arm 
is  \  ft.  long.     What  is  the  advantage?     What  lift  would  be  given  to 
the  piston  by  12  Ib.  of  force?     Make  a  sketch. 

8.  In  a  tack  lifter  the  force  arm  is  8  in.  long  and  the  weight  arm  is  \  in. 
long.     What  is  the  advantage?     What  lift  is  given  to  the  tack  by  20  Ib. 
of  force?     Make  a  sketch. 

9.  In  using  large  shears,  the  hand  applies  the  force  10  in.  from  the 
rivet,  and  the  cloth  is  i|  in.  from  the  rivet.     What  is  the  advantage? 
What  force  will  exert  60  Ib.  of  pressure  on  the  cloth  at  that  point? 
When  the  cloth  is  2  in.  from  the  rivet,  what  is  the  advantage?     What 
force  will  exert  60  Ib.  of  pressure?     When  the  cloth  is  3  in.  from  the 
rivet,  what  force  will  be  needed  to  exert  the  same  pressure? 

10.  The  handles  of  the  pliers  are  6  in.  long,  and  the  object  in  the 
jaws  is  f  in.  from  the  rivet.  What  is  the  advantage?  What  pressure 
will  be  exerted  by  10  Ib.  of  force?  Make  a  sketch.  If  the  object  is 
i \  in.  from  the  rivet,  what  is  the  advantage?  What  is  the  pressure 
with  10  Ib.  of  force? 


MECHANICS.     SOLIDS  II 

11.  When  is  a  lever  said  to  be  of  the  first  class?  of  the  second  class? 
of  the  third  class?     Make  a  diagram  of  each. 

12.  Name  three  appliances,  other  than  those  given  above,  which  are 
levers  of  the  second  class. 

13.  Name  three  lever  appliances  of  the  third  class  other  than  those 
given  above. 

14.  A  lemon  squeezer  has  force  arm  9  in.  and  weight  arm  3  in.     What 
is  the  advantage?     What  pressure  is  exerted  by  6  Ib.  of  force?     Neglect 
the  weight  of  upper  half  of  squeezer.     Make  a  sketch. 

15.  A  nut  cracker  has  a  force  arm  of  6  in.  and  the  center  of  the  nut 
is  2  in.  from  the  fulcrum.     What  is  the  advantage?     What  force  will 
be  required  to  exert  12  Ib.  pressure  on  the  nut?     Neglect  the  weight 
of  the  upper  half  of  nut  cracker.     Make  a  sketch. 

16.  On  a  fruit  press  the  distance  from  the  fulcrum  to  the  center  of  the 
hand  is  12^  in.,  and  the  pressure  is  exerted  2f  in.  from  the  fulcrum. 
What  is  the  advantage?     What  pressure  will  be  exerted  by  10  Ib.  of 
force?     Neglect  the  weight  of  the  upper  half  of  the  fruit  press.     Make 
a  sketch. 

17.  In  using  a  can  opener,  if  we  place  the  center  of  the  hand  5  in. 
from  the  fulcrum  and  the  knife  cuts  at  a  distance  of  f  in.  from  the  end, 
what  is  the  advantage?     WThat  is  the  cutting  pressure  with  10  Ib.  of 
force?     Neglect  the  weight  of  the  can  opener.     Make  a  sketch. 

18.  On  a  carving  knife  the  distance  from  the  little  finger   (fulcrum) 
to  the  thumb  and  forefinger  is  4  in.,  and  from  the  little  finger  to  the 
part  of  the  blade  at  which  the  cutting  is  done  is  12  in.      What  cutting 
pressure  will  be  exerted  by  15  Ib.  of  force?     Neglect  the  weight  of  the 
knife.     Make  a  sketch. 

19.  If  the  distance  from  the  center  of  the  bowl  of  a  tablespoon  to  the 
end  of  the  handle  is  6  in.,  and  the  distance  from  the  thumb  and  fore- 
finger to  the  end  of  the  handle  is  3  in.,  what  force  will  the  thumb  and 
forefinger  exert  to  lift   2  oz.  of  salt  in  the  bowl?  Neglect  weight  of 
spoon.     Make  a  sketch. 

20.  Why  is  the  force  always  greater  than  the  weight  in  levers  of  the 
third  class? 

21.  How  could  the  advantage  of  a  lever  of  the  third   class  be  in- 
creased ? 

22.  The  radius  of  the  drum  of  a  windlass  is  4  in.  and  the  crank  arm 
is  1 6  in.  long.     What  force  will  just  balance  100  Ib.  of  weight?     Make 
a  sketch. 

23.  The  radius  of  the  roll  in  a  wringer  is  i£  in.,  and  the  handle  is  10  in. 
The  hand  exerts  5  Ib.  of  force.     What  is  the  force  driving  the  clothes 
between  the  rolls?     Neglect  friction.     Make  a  sketch. 


12  PHYSICS   OF  THE  HOUSEHOLD 

24.  The  handle  of  a  bread  mixer  is  9  in.  long ;    the  radius  of  the 
blades  is  3  in.     What  is  the  pressure  on  the  blade,  if  the  hand  exerts  a 
force  of   12  Ib.  ?     Neglect  friction.     Make  a  sketch. 

25.  The  crank  arm  of  an  ice-cream  freezer  is  6  in.  long;    the  radius 
of  the  can  is  3  in.     What  is  the  moving  force  on  the  edge  of  the  can 
when  the  hand  exerts  a  force  of  15  Ib.?     Neglect  the  friction.     Make 
a  sketch. 

26.  The  radius  of  a  grate  is  3  in. ;   the  hand  applies  the  force  to  the 
handle  12  in.  from  the  center  of  the  axle.     What  force  will  give  a  mov- 
ing force  of  60  Ib.  to  the  top  of  the  grate?     Neglect  friction.     Make  a 
sketch. 

27.  In  your  own  home  measure  the  force  arm  and  weight  arm  and  calcu- 
late the  advantage  of  a  pair  of  scissors,  a  wringer,  and  at  least  two  other 
appliances. 


CHAPTER  II 
MECHANICAL  APPLIANCES    (continued) 

PULLEYS 

FOUR  systems  of  pulleys  are  shown  in  Fig.  9.    We  will  use 
these  to  illustrate  the  law  of   the  pulley.    This  law  is:  // 


(2)  (3) 

FIG.  9.  —  Four  pulley  systems. 


there  is  no  friction,  the  force  is  equal  to  the  weight  divided  by 
the  number  of  ropes  supporting  the  weight. 

In  (i)  the  weight  is  10  Ib.  and  there  is  one  rope  supporting  it. 
The  force  which  must  be  applied  to  the  spring  balance  to  support 
the  weight  is  10  Ib. 


14  PHYSICS  OF  THE  HOUSEHOLD 

In  (2)  there  are  two  ropes  supporting  a  total  weight  (pulley  + 
weight)  of  15  Ib. ;  each  rope  supports  only  half  this  weight  and 
the  force  necessary  is  half  the  weight,  or  7!  Ib. 

In  (3)  three  ropes  support  a  total  weight  of  15  Ib.  The  force 
needed  is  one  third  of  this,  or  5  Ib. 

In  (4)  four  ropes  support  a  total  weight  of  16  Ib.  The  force 
needed  is  one  fourth  of  this,  or  4  Ib. 

These  illustrate  the  law  of  the  pulley  stated  above. 

Window  pulleys  and  hanging  lamp  pulleys  are  familiar  ap- 
plications of  the  pulley  in  the  home. 

The  law  of  machines.  —  The  law  of  machines  is :  //  there  is 
no  friction,  the  weight  times  the  distance  the  weight  moves,  is  equal 
to  the  force  times  the  distance  the  force  moves.  We  can  illustrate 
this  law  by  means  of  the  pulleys  above.  In  (2),  Fig.  9,  the 
weight  is  15  Ib.,  and  the  force  supporting  it  is  y|  Ib.  If  we 
move  the  force  down  4  ft.,  the  weight  is  raised  only  2  ft.,  be- 
cause each  rope  supporting  the  weight  is  shortened  only  2  ft. 
Thus 

Weight  X  distance  weight  moves  =  force  X  distance  force  moves 

i5X  2  =  7J  X4 
30  =  30 

Similarly,  in  (3)  if  the  5  Ib.  of  force  is  moved  3  ft.,  the  15  Ib. 
of  weight  is  raised  only  i  ft.,  and 

Weight  X  distance  weight  moves  =  force  X  distance  force  moves 

IS  X  1  =  5  X3 
I5  =  i5 

Machines. — Any  contrivance  by  means  of  which  a  force 
applied  at  one  point  exerts  a  force  or  pressure  at  another  point 
is  called  a  machine.  Each  of  the  appliances  we  have  studied 
so  far  is  a  machine,  and  the  law  of  machines  applies  to  them,  as 
indeed  it  does  to  all  machines. 

Show  by  experiment  or  by  simple  examples  that  this  law 
applies  to  the  levers  and  the  wheel  and  axle. 


MECHANICAL  APPLIANCES  15 

THE  SCREW 

We  may  now  study  household  appliances  which  have  a  screw 
motion.    A  number  of  these  are  shown  in  Fig.   10;   namely, 


dealer  Clgmp 

FIG.   10.  —  Household  screw  appliances. 

the  bolt,  screw  nail,  and  jackscrew,  faucet,  fruit  press,  meat 
chopper,  sealer,  and  clamp. 


1 6  PHYSICS  OF  THE  HOUSEHOLD 

'An  appliance  with  a  screw  motion  is  used  when  it  is  necessary 
to  exert  a  great  pressure,  or  to  lift  a  great  weight  for  a  short 
distance.  For  example,  the  clamp,  bolt,  and  screw  nail  are 
used  to  hold  things  together  firmly.  Also,  the  screw  motion 
in  the  meat  chopper  gives  the  great  pressure  needed  to  force 
the  meat  through  the  holes  at  the  end.  Similarly,  the  screw  in 
the  fruit  press  gives  the  heavy  pressure  needed  to  force  the  juice 
out  of  the  fruit.  The  screw  top  of  the  sealer  exerts  great  pres- 
sure on  the  glass  cover,  etc.  Again,  when  it  is  necessary  to  lift 
a  great  weight,  such  as  a  house,  the  jackscrew  is  used. 

We  see,  then,  that  we  use  the  screw  motion  when  we  desire 
to  exert  a  great  pressure  or  lift  a  great  weight.  Let  us  now  get 
a  deeper  insight  into  this  screw  motion. 

The  law  of  machines  applies  to  the  screw.  —  The  jackscrew 
is  one  of  the  simplest  of  the  screw  appliances,  and  for  this 

reason  we  will  use  it  to 
illustrate  the  principle  of 
the  screw. 

We  can  understand  the 
screw  most  readily  by  means 
of  the  "law  of  machines." 
We  learned  in  the  last  sec- 
tion tha^  the  "  law  of 
machines  "  is,  —  if  there  is 

FIG.  1 1. -The  jackscrew.  no  Diction  in  a  machine : 

Weight  X  distance   weight 
moves  =  force  X  distance  force  moves. 

This  law  applies  to  all  screw  appliances. 

A  jackscrew  is  shown  in  Fig.  n.  The  weight  to  be  raised  is 
placed  on  the  head  of  the  screw,  and  the  force  is  applied  at  the 
end  of  the  handle.  When  the  force  moves  the  handle  through  one 
complete  turn,  the  weight  is  raised  a  distance  equal  to  the  dis- 
tance from  one  thread  on  the  screw  to  the  next.  The  distance 
from  one  thread  to  the  next  is  called  the  pitch  of  the  screw. 

The  jackscrew  in  Fig.  n  has  a  handle  28  in.  long  and  the 


v 


MECHANICAL  APPLIANCES  17 

pitch  is  J  in.,  let  us  calculate  the  weight  that  can  be  raised 
by  10  Ib.  of  force. 

In  one  complete  turn,  the  force  at  the  end  of  the  handle  moves 
a  distance  equal  to  the  circumference  of  a  circle  of  28  in.  radius. 

The  c  rcumference  of  a  circle  is  calculated  as  follows: 

Circumference  =  2  X  IT  X  radius 
TT  is  3.1416  or  *f-  (nearly) 

The  force  then  moves  2  X  *?•  X  28  =  176  in. 
When  the  screw  makes  one  complete  turn,  the  weight  is  raised 
a  distance  equal  to  the  pitch  of  the  screw,  in  this  case  f  in. 
The  force  is  10  Ib. 
The  law  of  machines  is,  if  there  is  no  friction : 

Weight  X  distance  weight  moves  =  force  X  distance  force  moves 
Weight  X  i  =  10  X  176 

Weight  =  10  X  176  X  4  =  7040  Ib. 

We  see,  then,  that  with  this  jackscrew  a  force  of  10  Ib.  raises 
the  enormous  weight  of  7040  Ib.,  or  over  3^  tons.  This  result 
is  obtained  on  the  assumption  that  there  is  no  friction.  There 
is  friction,  however,  in  every  machine,  and  in  the  jackscrew  it  is 
over  50  per  cent.  For  this  reason  the  weight  raised  by  10  Ib. 
of  force  would  actually  be  somewhat  less  than  half  of  7040  Ib., 
or  about  3000  Ib.  But  making  all  allowances  for  friction,  we 
see  that  the  screw  enables  us  to  lift  a  very  large  weight  with  a 
small  force. 

Advantage.  —  The  advantage  of  a  screw  is  found  by  dividing 
the  "  distance  the  force  moves  "  by  the  "  distance  the  weight 
moves,"  and  when  found  it  tells  how  many  times  greater  the 
weight  is  than  the  force  used.  For  example,  the  advantage 
of  the  jackscrew  above  is, 

Distance  force  moves       176  , 

-  =  -f-  =  704,  and 
Distance  weight  moves       \ 

Weight  _  7040  _ 
Force         10 
c 


1 8  PHYSICS   OF  THE  HOUSEHOLD 

That  is,  the  advantage  is  704,  and  the  weight  raised  is  704 
times  as  great  as  the  force  used. 

Screw  appliances.  —  The  clamp,  faucet,  meat  chopper,  and 
fruit  press  are  similar  to  the  jackscrew,  and  the  advantage  is 
found  as  stated  above. 

We  can  increase  the  advantage  in  each  case  by  lengthening 
the  handle.  For  example,  if  a  water  faucet  in  the  kitchen  turns 
with  difficulty,  it  can  be  made  to  turn  with  less  force  by  making 
the  handle  longer.  If  the  handle  is  made  twice  as  long,  the  force 
required  is  \  ;  if  it  is  made  three  times  as  long,  the  force  is  J ;  if 
10  times  as  long,  the  force  is  ^ ;  etc.  Similarly,  if  we  double 
the  length  of  the  handle  on  the  meat  chopper,  the  force  necessary 
is  one  half  as  great,  etc.  We  see  from  these  examples  and  those 
given  above  that  there  are  many  appliances  used  about  the  home 
which  would  require  less  force  if  their  handles  were  lengthened. 

The  sealer,  screw,  and  bolt  have  no  handles.  On  the  sealer 
the  force  is  applied  directly  to  the  edge  of  the  screw  top ;  on 
the  screw  the  force  is  applied  by  means  of  a  screw  driver.  In 
the  case  of  the  bolt,  the  wrench  used  is  the  handle. 

EXERCISES 

1.  Make  diagrams  of  systems  of  pulleys  in  which  (a)  one,  (b)  two,  (c) 
three,  (d)  four,  ropes  support  the  weight. 

2.  State  the  law  of  the  pulley. 

3.  Name  two  household  pulley  appliances. 

4.  A  window  weighs   30  Ib. ;    it    is  supported  by  two  counterpoise 
weights,  one  on  each  side,  working  over  pulleys.     What  must  each 
counterpoise  weigh  to  balance  the  window  ? 

5.  State  the  law  of  machines. 

6.  In  the  case  of   the  jackscrew  what  are  the  following:   the  weight; 
the  distance  the  weight  moves ;  the  force ;  the  distance  the  force  moves. 

7.  The  handle  of  a  jackscrew  is  14  in. ;    the  pitch  of   the  screw  is 
i  in.    What  weight  may  be  lifted  by  10  Ib.  of  force?     Neglect  friction. 

8.  A  clamp  handle  is  i  in.  long ;   the  pitch  of  the  screw  is  \  in.    What 
pressure  is  exerted  by  7  Ib.  of  force?     Neglect  friction.     Make  a  sketch. 

9.  A  fruit  press  handle  is  7  in.  long ;   the  pitch  is  \  in.  What  is  the 
pressure  on  fruit  from   10  Ib.  of  force?     Neglect  friction.     Make  a 
sketch. 


MECHANICAL  APPLIANCES  19 

/o.  On  a  sealer  the  radius  of  the  screw  cap  is  ij  in.,  and  the  pitch  ol 
the  screw  is  J  in.  What  pressure  will  be  exerted  on  the  glass  top  by 
14  Ib.  of  force?  Neglect  friction. 

ii.  The  handle  of  a  wrench  is  7  in.  long  from  the  center  of  the  jaws 
to  the  center  of  the  hand ;  the  pitch  of  the  thread  of  a  bolt  is  ^  in.  What 
is  the  pressure  exerted  by  the  nut  when  the  hand  is  exerting  14  Ib.  of 
force?  Neglect  friction. 


CHAPTER  III 
WORK 

IN  the  preceding  chapters  we  have  studied  various  mechanical 
appliances,  and  we  have  found  that,  with  the  exception  of  levers 
of  the  third  class,  the  appliances  enable  us  to  exert  greater 
pressure  than  the  force  we  use.  It  might  appear  from  this 
that  the  appliances  enable  us  to  obtain  something  for  nothing. 
We  shall  see,  in  our  study  of  work,  however,  that  this  is  not 
the  case. 

Work.  —  Work  is  done  when  a  force  is  exerted  through 
any  distance.  For  example,  when  a  window  is  raised,  work 
is  done  because  a  force  is  exerted  through  the  distance 
the  window  is  raised;  when  a  scuttle  of  coal  is  carried  up- 
stairs, work  is  done  because  a  force  is  exerted  through  a  distance 
equal  to  the  vertical  height  of  the  stairway. 

Before  we  go  further  we  should  distinguish  carefully  between 
the  terms  force  and  work. 

Force  is  that  which  tends  to  change  a  body's  state  of  rest,  or  of 
motion  in  a  straight  line.  That  is,  if  a  body  is  standing  still, 
anything  which  tends  to  set  it  in  motion  is  a  force;  also,  if  a 
body  is  moving,  anything  which  tends  to  stop  it  or  change  the 
direction  in  which  it  is  moving  is  a  force. 

Work  is  done  when  a  force  produces  or  destroys  motion,  and  the 
amount  of  work  done  is  equal  to  the  force  multiplied  by  the  dis- 
tance the  force  moves  in  the  direction  in  which  it  is  acting. 

Units  of  work. — The  common  units  of  work  aie  the  foot 
pound  and  the  gram  centimeter. 

A  foot  pound  of  work  is  the  amount  of  work  done  when  a 
pound  of  force  is  exerted  through  a  distance  of  one  foot .  If 

20 


WORK  21 

a  i  Ib.  weight  is  raised  one  foot  against  the  force  of  gravity,  one 
foot  pound  of  work  is  done.  If  10  Ib.  of  force  is  required  to 
raise  a  window,  and  the  window  is  raised  2  ft.,  20  foot  pounds 
of  work  are  done.  If  a  scuttle  of  coal  weighing  25  Ib.  is  carried 
up  a  stairway  8  ft.  high,  25  X  8  =  200  foot  pounds  of  work 
are  done.  In  this  case,  if  the  man  carrying  the  coal  weighs 
150  Ib.,  he  does,  in  addition,  150  X  8.=  1200  foot  pounds  of 
work  in  lifting  his  own  body  up  the  stairs.  If  a  force  does 
not  produce  motion,  there  is  no  work  done  in  the  scientific 
sense.  For  instance,  a  man  supporting  a  pound  weight  is 
exerting  a  force  of  i  Ib.  against  gravity,  but  he  is  not  doing 
work,  because  there  is  no  motion.  Also,  if  a  man  is  supporting  a 
weight  and  moves  it  horizontally,  there  is  no  work  done  because 
the  motion  is  not  in  the  direction  of  the  force ;  the  man  is  ex- 
erting i  Ib.  of  force  upward,  and  to  do  work  he  must  move 
the  weight  in  the  direction  the  force  is  acting ;  that  is,  upward. 

A  gram  centimeter  of  work  is  done  when  a  force  of  i  gram  is 
exerted  through  a  distance  of  i  cm.  If  a  gram  weight  is 
raised  i  cm.  against  the  force  of  gravity,  i  gram  centimeter  of 
work  is  done,  etc. 

The  law  of  work.  —  Now  that  we  understand  the  meaning  of 
the  term  work,  we  are  in  a  position  to  show  that  we  do  not  ob- 
tain "  something  for  nothing  "  when  we  use  any  appliance. 
We  shall  see  that  in  any  appliance  "if  there  is  no  friction,  the 
work  we  obtain  from  an  appliance  is  just  equal  to  the  work  we 
put  into  it."  This  is  the  law  of  work. 

We  will  illustrate  the  meaning  of  this  law,  by  showing  how  it 
applies  to  levers  and  pulleys. 

The  lever,  Fig.  i,  has  a  force  arm  16  in.  long  and  a  weight 
arm  8  in.  long.  The  force  is  i  Ib.  and  the  weight  2  Ib.  If  we 
move  the  force  down  i  ft.,  the  weight  is  raised  only  \  ft.  be- 
cause the  weight  arm  is  only  one  half  as  long  as  the  force  arm. 

The  work  we  obtain  is  2  X  \  =  i  foot  pound,  and  this  is  equal 
to  the  work  we  do ;  namely,  i  X  i  =  i  foot  pound.  That  is, 
the  law  of  work  applies  to  the  lever. 


22  PHYSICS  OF  THE  HOUSEHOLD 

The  pulley  (4),  Fig.  9,  has  four  ropes  supporting  a  weight  of 
1 6  Ib.  The  force  required  is  4  Ib.  If  the  force  moves  down 
8  ft.,  the  weight  is  raised  only  2  ft.  The  work  obtained  is 
1 6  X  2  =32  foot  pounds,  and  this  is  equal  to  the  work  we  do ; 
namely,  4  X  8  =  32  foot  pounds.  That  is,  the  law  of  work 
applies  to  the  pulley. 

Law  of  work  holds  for  all  appliances.  —  We  have  considered 
only  two  appliances,  but  the  law  of  work  holds  for  all  appli- 
ances :  "  If  there  is  no  friction,  the  work  obtained  from  an  ap- 
pliance is  equal  to  the  work  put  into  it." 

This  is  in  reality  the  "  law  of  machines,"  stated  in  terms  of 
work.  The  law  of  machines  states  that  "  if  there  is  no  friction, 
the  force  times  the  distance  the  force  moves  equals  the  weight 
times  the  distance  the  weight  moves."  The  "  force  times  the 
distance  the  force  moves  "  is  the  work  put  into  a  machine,  and 
"  the  weight  times  the  distance  the  weight  moves  "  is  the  work 
obtained  from  a  machine.  We  see  then  that  the  law  of  machines 
and  the  law  of  work  are  simply  different  ways  of  stating  the  same 
law. 

Friction.  —  In  every  appliance  there  is  a  certain  amount  of 
friction  or  resistance  to  motion,  and  therefore  part  of  the  work 
we  put  into  the  appliance  is  used  up  in  moving  against  this 
friction.  As  a  result,  the  work  we  obtain  from  any  appliance 
in  actual  use  is  never  quite  equal  to  that  which  we  put  into  it. 

The  advantage  of  the  appliances  we  have  studied  is,  that  in 
general  they  enable  us  to  lift  a  large  weight  or  exert  a  great 
pressure  by  the  exertion  of  a  small  amount  of  force.  In  every 
case,  however,  the  work  we  actually  do  is  greater  than  the  work 
we  obtain  by  means  of  the  appliance. 

EXERCISES 

1.  Define  work,  foot  pound,  and  gram  centimeter. 

2.  A  girl  weighing  loolb.  walks  up  a  stairway  20  ft.  high.    How  much 
work  does  she  do? 

3.  A  boy  pulls  a  sled  with  a  force  of  5  Ib.  through  a  distance  of  100  yd. 
How  much  work  does  he  do? 


WORK  23 

4.  A  man  carries  a  scuttle  of  coal  weighing  20  Ib.  up  a  stairway  15  ft. 
high.     How  much  work  does  he  do? 

5.  A  man  carries  a  2o-lb.  scuttle  of  coal  horizontally  from  the  cellar 
door  to  the  stove.     How  much  work  does  he  do? 

6.  A  boy  weighing  120  Ib.  jumps  over  a  fence  4  ft.  high.    How  much 
work  does  he  do  in  going  up?     How  much  work  does  the  attraction  of 
the  earth  do  in  pulling  him  down  ? 

7.  A  horse  pulls  on  a  load  with  a  force  of  100  Ib.  and  moves  it  i  mile. 
How  much  work  does  the  horse  do? 

8.  The  force  arm  of  a  pump  handle  is  3  ft.  long,  the  weight  arm  £  ft. 
long;    10  Ib.  of  force  moves  the  handle  i  ft.      If  there  is  no  friction, 
what  is  the  weight?  How  far  is  it  moved?     How  much  work  is  put  into 
the  appliance,  and  how  much  is  obtained  from  it? 

9.  The  handle  of  a  wringer  is  8  in.  long,  the  radius  of  the  roll  is  i  in. 
In  wringing  out  a  garment  the  hand  exerts  10  Ib.  of  force  and  moves 
1 6  ft.     If  there  is  no  friction,  how  much  work  is  put  into  the  wringer 
and  how  much  is  obtained  from  it? 


CHAPTER  IV 
MEASUREMENT 

SCIENTIFIC  knowledge  is  our  ordinary  knowledge  made  exact 
and  extended.  In  order  to  make  knowledge  exact,  we  must 
measure  everything  involved.  You  will  notice  that  in  the  sec- 
tions above  we  made  our  knowledge  of  household  appliances 
more  exact  by  measurement.  We  measured  force  arm,  weight 
arm,  force,  weight,  etc.  We  will  now  make  experiments  in 
order  to  gain  some  experience  in  measurement,  particularly  in 
the  measurements  required  about  the  home. 

Experiment  i.    Weights  and  measures.1 

Teaspoon,  tablespoon,  and  cup.     Use  sugar,  salt,  or  flour,  to  find, 

(1)  the  number  of  level  teaspoons  in  i  level  tablespoon  = 

(2)  the  number  of  level  tablespoons  in  i  level  cup  = 
Cup,  pint,  quart,  gallon.     Use  water  to  find, 

(3)  the  number  of  cups  in  i  pt.  = 

(4)  the  number  of  pints  in  i  qt.  = 

(5)  the  number  of  quarts  in  i  gal.  = 

Cubic  inches  in  quart  or  gallon.  Measure  the  inside  depth  and  diam- 
eter of  the  quart  or  gallon  measure  and  calculate, 

(6)  the  number  of  cubic  inches  in  i  qt.  = 

(7)  the  number  of  cubic  inches  in  i  gal.  = 

[  Note.  —  The  volume  of  cylinder  =  TT  X  (radius) z  X  depth,  where 
TT  =  3.1416  or  z?-  (nearly).] 

Weight  of  i  qt.  and  of  i  gal.  of  water.  Weigh  the  quart  and  gallon 
measures  when  empty  and  then  when  filled  with  water,  to  determine, 

(8)  the  weight  of  i  qt.  of  water  =  Ib. 

(9)  the  weight  of  i  gal.  of  water  =  Ib. 
Dry  Measure.     Use  oats  or  wheat  to  find, 

(10)  the  number  of  quarts  in  i  pk.  = 
(n)  the  number  of  pecks  in  i  bu.  = 

1  See  tables  of  weights  and  measures,  page  303  et  seq. 
24 


MEASUREMENT  .  25 

Experiment  2.     Metric  weights  and  measures. 

Centimeter,  inch,  and  foot.  Draw  on  paper  a  line  10  in.  long,  measure 
it  in  centimeters  and  calculate, 

(1)  i  in.  =          cm. 

Draw  a  line  i  ft.  long  and  measure  it  in  centimeters. 

(2)  i  ft.  =          cm. 

Cubic  centimeters  in  1 1.  Measure  in  centimeters  the  inside  depth  and 
diameter  of  a  cylindrical  liter  measure  and  calculate, 

(3)  i  1.  =          c.c. 

Weight  of  i  I.  of  water  in  grams.  Weigh  in  grams  the  liter  measure 
when  empty  and  when  full  of  water,  to  determine 

(4)  i  1.  of  water  =          grams. 

Liter  and  quart.  Fill  a  liter  measure  with  water  and  pour  the  water 
into  a  quart  measure  to  determine  roughly 

(5)  i  qt.  =          liters. 

Kilogram  and  pound.  Weigh  a  kilogram  weight  in  pounds  to  deter- 
mine 

(6)  i  kg.  =        Ib. 

WEIGHTS  AND  MEASURES 

In  English-speaking  countries  there  are  two  systems  of 
weights  and  measures,  the  common  system  and  the  metric 
system.  In  the  common  system,  which  is  used  in  commerce, 
the  units  are  the  yard,  the  gallon,  and  the  pound.  In  the 
metric  system,  which  is  used  in  all  scientific  work,  the  units 
are  the  meter,  the  liter,  and  the  kilogram.  We  shall  now  de- 
vote a  little  time  to  the  study  of  these  two  systems. 

Advantages  and  disadvantages  of  the  common  system.  - 
The  common  system  has  one  great  advantage ;  namely,  we  are 
all  acquainted  with  it.     It  has,  however,  many  disadvantages. 
These  may  be  summarized  as  follows : 

1.  The  multiples  are  irregular.     For  example,  12  in.  =  i  ft. ; 
3  ft.  =  i  yd. ;  5^  yd.  =  i  rd. 

2.  There  are  no  simple  relations  between  the  units  of  length, 
volume,  and  weight.     For  example,  in  the  United  States  i  qt.  = 
57.75  cu.  in.,  and  i  qt.  of  water  weighs  2.082  Ib.     In  Great 
Britain  and  Canada  i  qt.  =  69.318  cu.  in.,  and  i  qt.  of  water 
weighs  2.5  Ib. 


26  PHYSICS  OF  THE  HOUSEHOLD 

3.  Some  units  have  the  same  name,  but  different  values. 
For  example,  the  ounce  and  pound  Troy  are  different  from  the 
ounce  and  pound  avoirdupois. 

4.  The   units   are   not   the   same   as   those   used   in   other 
countries. 

The  common  system  is  the  result  of  centuries  of  usage,  and, 
like  the  English  system  of  coinage,  is  very  awkward.  In  the 
United  States  and  Canada  a  decimal  system  of  coinage  has  been 
adopted,  but  the  old  system  of  weights  and  measures  is  still 
used. 

History  of  the  metric  system.  —  At  the  time  of  the  French 
Revolution  the  weights  and  measures  used  in  France  were  in  a 
worse  condition  than  ours  are  at  the  present  time,  because  the 
units  had  different  values  in  different  parts  of  the  country.  To 
remedy  this  condition  of  affairs,  the  French  government  ap- 
pointed a  commission  to  devise  a  new  system  of  weights  and 
measures.  It  was  the  aim  of  the  commission  to  obtain  a  sys- 
tem in  which  all  measures  should  be  based  upon  one  invariable 
unit  and  in  which  the  simplest  possible  relations  should  exist 
between  the  different  units  of  the  system. 

The  unit  of  length  adopted  was  the  meter,  and  all  other  units 
were  based  upon  this  unit  of  length.  The  commission  attempted 
to  make  the  meter  equal  in  length  to  the  ten-millionth  part  of 
the  quadrant  of  the  earth's  meridian.  This  was  to  be  the  in- 
variable unit.  The  earth's  quadrant  was  measured  and  the 
meter  was  made  equal  to  the  one  ten-millionth  part  of  this 
length.  The  meter  measure  was  made  of  platinum,  and  the 
distance  between  two  transverse  scratches  on  this  bar  was  made 
equal  to  one  meter.  It  was  found  later  that  an  error  had  been 
made  in  the  measurement  of  the  quadrant  of  the  earth's  me- 
ridian. It  was  decided,  however,  not  to  change  the  length  of 
the  meter,  and  the  meter  is  now  defined,  not  as  the  ten-mil- 
lionth part  of  the  earth's  quadrant,  but  as  the  distance  between 
the  two  scratches  on  the  bar  mentioned  above. 

The  commission  made  simple  relations  between  the  units  of 


MEASUREMENT  27 

length,  volume,  and  mass  as  follows.  The  unit  of  length  is  the 
meter.  The  unit  of  volume  is  one  tenth  meter  cubed;  it  is 
called  a  liter.  The  unit  of  mass  is  a  piece  of  platinum  which 
has  the  same  weight  as  a  liter  of  water  at  4°  C.  This  is  called  a 
kilogram. 

Advantages  of  the  metric  system.  —  The  metric  system  has 
the  following  advantages: 

1.  The  multiples  are  simple;  namely,  ten,  one  hundred,  and 
one  thousand. 

2.  There  are  simple  relations  between  the  units  of  length, 
area,  volume,  and  mass. 

3.  The  system  is  used  in  scientific  work  in  all  civilized  coun- 
tries.    It  is  used  also  in  trade  in  many  European  countries, 
and  is  being  so  used  more  and  more  in   English-speaking 
countries. 

The  common  system  of  weights  and  measures  is  expensive 
because  an  unnecessary  amount  of  time  is  needed  to  make  cal- 
culations with  it.  The  difference  between  the  two  systems 
in  this  respect  can  be  illustrated  by  two  simple  problems  as 
follows  : 

i.  If  a  pound  of  meat  costs  20  cents,  what  is  the  cost  of  6 
oz.  of  meat  ? 


$  X  20  =  7^  cents. 

2.  If  a  kilogram  of  meat  costs  50  cents,  what  is  the  cost  of 
^  of  a  kilogram  ? 

Ans.   -f$  X  50  =  30  cents. 

To  solve  the  first  problem  we  need  a  pencil  and  paper,  while 
the  last  can  be  solved  mentally.  This  illustrates  the  differ- 
ence between  the  two  systems.  To  make  calculations  in  the 
common  system  requires  more  time,  and  since  time  is  worth 
money,  the  cost  each  year  to  the  country  using  it  is  very  great. 
The  metric  system  is  being  used  more  and  more  in  trade,  and  it 
is  hoped  that  those  who  read  this  book  will  aid  its  introduction 
by  advocating  it  and  using  it  where  possible. 


28  PHYSICS  OF  THE  HOUSEHOLD 

UNITS 

Standard  units.  —  When  a  unit  becomes  legalized  by  statute 
or  custom  it  is  called  a  standard  unit.  For  example  in  English- 
speaking  countries  the  standard  units  of  length  and  mass  are 
the  yard  and  the  pound. 

The  standard  yard  is  defined  as  the  distance  at  62°  F.  be- 
tween the  lines  on  two  gold  plugs  inserted  in  a  bronze  bar  de- 
posited in  the  office  of  the  Exchequer  in  London.  The  standard 
foot  is  one  third  the  standard  yard. 

The  standard  pound  of  mass  is  a  cylinder  of  platinum  depos- 
ited in  the  office  of  the  Exchequer  in  London. 

In  the  metric  system  the  standard  units  of  length  and  mass 
are  the  meter  and  the  kilogram.  These  have  been  denned 
above. 

The  standard  unit  of  time  in  both  systems  is  the  second. 
There  are  24  hr.  in  one  day,  60  min.  in  one  hour,  and  60  sec.  in 
one  minute,  therefore  there  are  24  X  60  X  60  =  86,400  sec. 
in  one  day.  The  standard  second  is  the  g^l^iJ-th  part  of  a  mean 
solar  day. 

Fundamental  and  derived  units.  —  All  physical  measure- 
ments may  be  reduced  to  measurements  of  length,  mass,  and 
time.  For  example,  the  measurement  of  an  area  consists  of 
measurements  of  length;  the  measurement  of  a  volume  con- 
sists of  measurements  of  length ;  the  measurement  of  velocity 
consists  of  a  measurement  of  length  and  a  measurement  of  time. 

The  units  of  length,  mass,  and  time  are  called  fundamental 
units.  The  units  of  area,  volume,  velocity,  etc.,  are  called 
derived  units,  because  they  are  derived  from  measurements  with 
the  fundamental  units. 

Mass  and  weight.  —  In  the  sections  above  we  have  used  the 
two  terms  mass  and  weight.  It  is  necessary  to  distinguish 
between  these.  The  mass  of  a  body  is  denned  as  the  quantity 
of  matter  the  body  contains.  The  weight  of  a  body  is  the 
measure  of  the  earth's  attraction  for  the  body. 


MEASUREMENT  29 

The  difference  between  mass  and  weight  may  be  readily  under- 
stood from  the  following  example:  Let  us  suppose  that  a  body 
could  be  raised  one  million  -  miles  above  the  earth.  Would 
there  be  any  change  in  its  mass  or  weight?  There  would  be 
no  change  in  its  mass,  because  the  quantity  of  matter  in  the 
body  would  remain  the  same.  There  would,  however,  be  a 
change  in  weight.  The  earth  would  attract  the  body  with  less 
force,  and  therefore  the  weight  of  the  body  would  be  less. 

Household  measurements.  —  The  measurements  most  often 
made  in  the  home  are  those  of  volume,  weight,  time,  and  tem- 
perature. For  example,  a  good  cook  in  making  a  cake  measures 
the  volume  or  weight  of  the  ingredients.  She  mixes  these  in- 
gredients and  places  them  in  an  oven  at  a  certain  temperature 
and  leaves  them  there  for  a  certain  time. 

As  was  stated  above,  the  chief  difference  between  ordinary 
knowledge  and  scientific  knowledge  is  that  the  latter  is  more 
exact.  Knowledge  is  made  exact  by  measurement.  A  cook, 
to  do  exact  or  scientific  work,  must  have  in  the  kitchen :  a  cup 
of  known  volume  to  measure  volume,  a  balance  to  measure 
weight,  a  clock  to  measure  time,  and  an  oven  thermometer  to 
measure  temperature. 

EXERCISES 

1.  How  many  teaspoonfuls  make  one  tablespoonful,  and  how  many 
tablespoonfuls  make  one  cup? 

2.  How  many  cups  in  i  pt. ;  pints  in  i  qt. ;   quarts  in  i  gal.? 

3.  How  many  cubic  inches  are  there  in  i  qt.?  in  i  gal.? 

4.  What  is  the  weight  of  i  qt  of  water?  of  i  gal.  of  water? 

5.  How  many  quarts  are  there  in  i  pk.  ?  pecks  in  i  bu.? 

6.  How  many  centimeters  are  there  in  i  in.  ?  in  i  ft.  ? 

7.  How  many  cubic  centimeters  are  there  in  i  1.  ? 

8.  What  is  the  weight  in  grams  of  i  1.  of  water;   of  i  c.c.  of  water? 

9.  How  many  liters  are  there  in  i  qt.  (about)  ? 

10.  How  many  pounds  are  there  in  i  kg.? 

11.  State  the  advantages  and  disadvantages  of  the  common  system 
of  weights  and  measures. 

12.  State  the  advantages  of  the  metric  system. 


CHAPTER  V 
MECHANICS.     LIQUIDS 

WATER  SUPPLY 

City  water  supply.  —  One  of  the  most  important  problems 
which  a  city  government  has  to  meet  is  that  of  providing  an 
ample  supply  of  water  for  all  parts  of  the  city.  There  are  a 


FIG.   12.  —  Water  supply  system  for  towns  and  cities. 

number  of  ways  in  which  this  is  done.  One  of  these  is  illustrated 
in  Fig.  12.  A  large  pump  in  the  municipal  pumping  station 
draws  water  from  a  river  or  lake,  and  forces  it  into  a  large 
standpipe.  From  the  standpipe  the  water  runs  by  gravity 
through  the  mains  and  submains  to  the  houses,  hydrants,  etc. 

If  the  city  is  situated  near  a  hill,  the  usual  practice  is  to  build 
a  large  reservoir  on  the  hill  and  pump  the  water  into  this,  in- 
stead of  into  a  standpipe.  The  water  then  runs  by  gravity 
from  the  reservoir  through  the  mains  and  submains. 

In  many  cities  neither  standpipe  nor  reservoir  is  used;  the 
water  is  pumped  directly  into  the  mains.  In  this  case  the  en- 
gine which  drives  the  pump  is  equipped  with  an  automatic  regu- 

30 


MECHANICS.     LIQUIDS  31 

lator  which  is  operated  by  the  water  pressure  in  the  mains. 
This  regulator  is  so  arranged  that  the  water  pressure  in  the 
mains  is  kept  within  a  few  pounds  of  a  certain  amount. 

If  the  city  is  situated  near  a  hill  on  which  there  is  a  lake  of 
sufficient  size,  above  the  level  of  the  highest  buildings,  a  pipe 
line  is  laid  from  the  lake  to  the  city  mains.  This  is  the  most 
satisfactory  system  because  the  water  runs  by  gravity  and  there 
is  no  outlay  for  pumping. 

Water  supply  for  country  homes.  —  There  are  a  number  of 
methods  of  supplying  country  homes  with  running  water.  One 
of  these  is  illustrated  in  Fig.  13.  The  water  from  a  well,  cistern, 
river,  or  lake  is  pumped  in  o  an  elevated  tank  from  which  it 
runs  by  gravity  to  the  various  fixtures.  The  elevated  tank 


FIG.   13.  — Water  supply  system  for  country  homes. 


32  PHYSICS  OF  THE  HOUSEHOLD 

is  placed  in  the  attic  of  the  house,  in  the  hayloft  of  the  barn, 
on  a  near-by  hill,  or  on  a  tower.  The  pumping  is  done  by  hand 
or  by  some  form  of  power  appliance,  such  as  a  windmill  or  gaso- 
line engine.  In  Fig.  13  the  windmill  operates  a  pump  and 
forces  water  from  the  well  to  an  elevated  tank  on  a  tower.  The 
water  from  the  tank  runs  by  gravity  to  the  house,  fountain, 
stable,  and  to  the  troughs  in  the  fields. 

If  the  house  is  near  a  stream  which  has  a  fall  of  at  least 
ij  ft.,  the  water  can  be  pumped  into  the  elevated  tank  by 
means  of  a  hydraulic  ram.  The  hydraulic  ram  is  described 
on  page  64. 

If  the  house  is  near  a  spring  which  has  an  elevation  greater 
than  that  of  the  highest  house  tap,  the  water  can  be  piped  di- 
rectly from  the  spring  to  the  house  supply  pipes.  In  this  case  a 
tank  in  the  house  is  unnecessary ;  but  if  the  spring  is  small,  a 
storage  tank  just  below  the  spring  may  be  advisable. 

Another  method  of  supplying  water  to  country  homes  is 
by  means  of  a  pneumatic  tank.  This  method  is  described  on 
page  62. 

Wells.  —  The  source  of  all  water  supply  is  the  water  which 
falls  upon  the  earth  as  rain  or  snow.  Part  of  this  rain  or  snow 


WELL 


o  ~=LP  :_  b_L     >.*L9-  -G  ROU-N  5 


FIG.   14.  —  Source  of  water  in  wells  and  spring 


water  sinks  into  the  ground  and  finds  its  way  underground  to 
the  neighboring  streams.  When  water  enters  the  ground  it 
sinks  until  it  comes  to  a  nonporous  stratum,  and  then  flows  by 
gravity  along  the  surface  of  this  stratum  until  it  finds  an  outlet 


MECHANICS.     LIQUIDS 


33 


in  a  stream,  lake,  or  spring.  This  moving  water  is  called  ground 
water,  and  its  surface  is  called  the  ground  water  level.  The  sur- 
face well  shown  in  Fig.  14  is  filled  with  ground  water.  The 
water  is  flowing  towards  the  spring  C  through  the  porous  stra- 
tum and  above  the  nonporous  stratum.  The  well  is  rilled  as 
high  as  the  ground  water  level.  The  ground  water  level  rises 
in  wet  weather  and  falls  in  dry  weather ;  and  the  water  level  in 
the  well  rises  and  falls  with  it.  Since  the  ground  water  is  mov- 
ing, the  water  in  the  well  is  constantly  changing.  Water  flows 
in  on  the  side  A  and  out  on  the  side  B.  This  illustrates  how 
surface  wells  and  springs  are  supplied  with  water. 

/Artesian  wells.  —  The  conditions  producing  an  artesian  or 
flowing  well  are  illustrated  in  Fig.  15.    The  earth's  surface  in 


FIG.  15.  —  Conditions  which  produce  artesian  or  flowing  wells. 

many  parts  is  made  up  of  distinct  strata ;  some  of  these  are 
porous  to  water,  and  others  are  nonporous.  Four  strata  are 
represented  in  Fig.  15.  Two  are  porous  and  two  nonporous. 
These  strata  may  be  hundreds  of  miles  in  extent.  Rain  which 
falls  on  the  hills,  between  the  points  A  and  B,  sinks  by  gravity 
into  the  second  porous  stratum.  This  stratum  has  an  imper- 
vious stratum  above  it  and  another  beneath  it.  If  the  hollow 
is  bowl-shaped,  the  water  fills  the  second  porous  stratum  until 
it  finds  an  outlet  at  some  point  C.  The  water  in  the  second 
porous  stratum  is  then  at  the  level  of  the  dotted  line.  If  a  well 
is  sunk  at  E  into  the  second  porous  stratum,  the  water  rises  in 
the  well  to  the  level  of  the  dotted  line.  If  a  well  is  sunk  at  Dt 


34 


PHYSICS  OF  THE  HOUSEHOLD 


the  water  rises  above  the  surface  and  the  well  is  a  flowing  well. 
If  there  were  no  friction  in  the  soil  and  in  the  casing  of  the  well, 
the  wat^r  would  shoot  up  as  high  as  the  dotted  line.  Since 
there  is  a  great  deal  of  friction,  however,  particularly  in  the  soil, 
the  water  does  not  rise  to  this  height. 

Well  on  a  hillside.  —  Water  which  falls  upon  the  hills  and 
sinks  into  the  earth  flows  underground  by  gravity  down  the 
hillsides  to  the  valleys  and  down  the  valleys  to  the  streams. 
In  some  parts  of  the  country  a  supply  of  running  water  is  secured 
by  sinking  a  well  on  a  hillside  (see  Fig.  16).  The  ground  water 
fills  this  well  to  the  ground  water  level  at  that  point.  If  this 


FIG.  1 6.  —  Water  supply  from  a  hillside  well. 

level  is  above  the  highest  house  tap  and  a  pipe  is  laid  from  the 
well  to  the  house,  the  water  runs  from  the  well  to  the  highest 
house  tap  by  gravity. 

LAWS   RELATING  TO   PRESSURE   IN  LIQUIDS 

Pressure  in  liquids.  —  When  a  tap  is  opened  in  a  house  sup- 
plied with  running  water,  the  water  comes  out  with  more  or 
less  force,  according  to  the  pressure  upon  it.  The  pressure 
may  be  due  to  the  force  exerted  on  the  water  by  the  piston  of 
the  pump  at  the  city  pumping  station,  or  it  may  be  due  to  the 
force  of  gravity  on  the  water  in  a  standpipe,  reservoir,  or  ele- 
vated tank.  In  order  to  get  a  deeper  insight  into  this  and  into 
other  properties  of  liquids,  we  will  now  study  the  laws  relating 
to  pressure  in  liquids. 

One  cubic  foot  of  water  weighs  approximately  62.5  Ib.  If 
then  a  tank  is  filled  with  water  to  a  depth  of  i  ft.,  the  pres- 


MECHANICS.     LIQUIDS  35 

sure  on  each  square  foot  of  the  bottom  is  62.5  Ib.  If  the  tank 
is  filled  to  a  depth  of  4  ft.,  the  pressure  on  each  square  foot  is 
4  X  62.5  =  250  Ib.  That  is,  the  pressure  on  a  given  surface 
is  proportioned  to  the  depth. 

If  a  tank  has  a  bottom  6  sq.  ft.  in  area  and  it  is  filled  to  a 
depth  of  4  ft.,  the  total  pressure  on  the  bottom  is  6X4X62. 5  = 
1500  Ib.  That  is, 

P  =  a  X h Xd 

P  =  6  X  4  X  62.5  =  1500  Ib. 

where  P  =  total  pressure,  a  =  area  of  surface,  h  =  height  of 
water,  d  =  density  of  water. 

That  is,  the  total  pressure  of  a  liquid  on  a  surface  is  equal  to 
the  area  of  the  surface  times  the  depth  of  the  liquid  times  the 
density  of  the  liquid. 

Example.  —  The  area  of  the  base  of  a  tank  is  200  sq.  cm. ; 
the  depth  of  the  water  is  60  cm. ;  what  is  the  pressure  on  the 
base  ?  Note,  i  c.c.  of  water  weighs  i  g. 

P  =  a  X  h  Xd 

P  =   200   X  60   X  I    =   1 2,000  g. 

The  pressure  upward  at  any  point  in  a  liquid  is  equal  to  the  pres- 
sure downward  at  this  point.  If  a  glass  lamp  chimney  A, 
Fig.  17,  is  fitted  with  a  thin  ground  glass  bottom  O  which  is 
held  over  one  end  with  a  thread  C,  while 
this  end  is  placed  in  water,  it  is  found  that 
the  bottom  remains  on  when  the  thread  is 
released.  This  shows  that  water  exerts 
pressure  upward. 

If  now  water  is  poured  into  the  chimney, 
the  bottom  remains  on  until  the  level  of 
the  water  inside  the  chimney  is  the  same 
as  the  level  outside.  This  shows  that  the 
pressure  upward  in  the  water  is  equal  to 
the  pressure  downward  of  the  column  of  FlG  I7^_Liquids  exert 
liquid  inside  the  chimney.  In  other  pressure  upwards. 


PHYSICS  OF  THE  HOUSEHOLD 


words,  in  liquids  the  pressure  upward  is  equal  to  the  pressure 

downward  at  any  depth. 
Pressure  at  any  point  in  a  liquid  is  the  same  in  all  directions. 

—  The  bent  tubes,  Fig.  18,  are  filled  with  mercury  to  the  same 

depth.  The  short  arm  of 
each  tube  is  open.  The 
short  ends  point  upward, 
sidewise,  and  downward, 
respectively.  When  these 
tubes  are  lowered  to  the 
same  depth  in  a  liquid 
the  mercury  shows  the 
same  pressure  in  each 
tube.  This  experiment 
shows  that  at  any  depth 
in  a  liquid  the  pressure  of 
the  liquid  is  the  same  in 
all  directions,  downward, 
sidewise,  and  upward. 

This  explains,  for  ex- 
ample, why  the  pressure 
at  a  tap  is  the  same,  no 
matter  whether  it  is  at- 
tached to  the  top,  bottom, 
or  side  of  a  pipe,  provided 
that  in  each  position  it  is 
the  same  distance  below 
the  water  surface. 

The  hydrostatic  para- 
dox. Pressure  independ- 
ent of  volume.  —  A 

striking  fact  regarding  liquid  pressure  is  shown  by  the  following 

experiment: 

A  cone-shaped  vessel,  i,Fig.  19, is  arranged  with  a  loose  bottom 

AB,  held  up  by  weights  on  the  pan  of  the  balance  (not  shown) ; 


FIG.  18.  —  The  pressure  at  any  point  in  a 
liquid  is  the  same  in  all  directions. 


MECHANICS.     LIQUIDS 


37 


the  weights  are  so  adjusted  that  the  pressure  of  the  water  forces 
the  base  off  when  it  reaches  a  certain  height. 

If  then  the  cone-shaped  vessel  is  replaced  by  the  top  shown  in 
2,  which  has  a  much  smaller  volume,  it  is  found  again  that  the 
base  is  forced  off  when  the  water  reaches  the  same  height. 

Also,  when  2  is  replaced  by  the  top  3  with  a  still  smaller  vol- 
ume, it  is  found  again  that  the  base  is  forced  off  when  the  level 
of  the  water  reaches  the  same  height.  This  is  called  the  hydro- 
static paradox,  because,  although  the  weight  of  water  is  very 


2  3 

FIG.  19.  —  The  hydrostatic  paradox. 


different  in  each  case,  the  pressure  on  a  given  base  is  the  same 
for  equal  depths  of  water. 

We  conclude,  then,  that  the  pressure  exerted  by  a  liquid  on  any 
surface  depends  only  upon  the  area  of  the  surface,  the  depth  and 
density  of  the  liquid,  and  not  at  all  upon  the  volume  of  the  liquid. 

This  is  shown  also  in  4.  The  water  is  at  the  same  level  in 
each  top,  although  the  volume  of  liquid  in  each  is  different. 

We  learn  from  this,  for  example,  that  the  pressure  per  square 
inch  at  any  faucet  is  independent  of  the  size  or  shape  of  the 
storage  tank  in  the  attic  or  elsewhere.  It  depends  only  upon 
the  distance  the  tap  is  below  the  surface  of  the  water,  and  upon 
the  density  of  the  water. 

Pascal's  law.  —  The  apparatus  shown  in  Fig.  20  is  a  syringe 
with  a  number  of  nozzles.  If  it  is  filled  with  water  and  the  piston 
is  pushed  in,  pressure  is  exerted  upon  the  liquid,  and  some  of 
the  water  flows  out  at  each  nozzle. 

Since  the  pressure  is  exerted  in  the  direction  of  the  end  nozzle, 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  20. —  The  streams 
are  of  the  same  length. 


we  should  naturally  expect  that  the  stream  from  this  would  be 
the  longest.  If,  however,  we  place  the  nozzles  in  the  same 
horizontal  plane  and  make  the  experiment,  we  find  that  the 
streams  from  the  nozzles  are  all  of  the  same  length.  From 
this  we  see  that  when  pressure  is  exerted 
on  a  confined  liquid,  it  is  transmitted 
equally  in  all  directions. 

If  each  nozzle  is  fitted  with  a  pressure 
gauge  and  a  pressure  of  10  Ib.  per  square 
inch  is  exerted  on  the  piston,  we  find  that 
each  nozzle  registers  a  pressure  of  10  Ib. 
per  square  inch.  This  shows  that  the  pres- 
sure exerted  on  a  confined  liquid  is  trans- 
mitted equally  and  undiminished  in  all 
directions. 

From  experiments  such  as  these  the 
French  physicist  Pascal  (1623-1662)  was 
led  to  the  discovery  of  a  law  of  nature  applying  to  liquids, 
named  after  the  discoverer,  Pascal's  law;  namely,  "Pressure 
exerted  on  a  confined  liquid  is  transmitted  equally  and  un- 
diminished in  all  directions." 

Applications  of  Pascal's  law.  — In  Fig.  21,  two  cylinders  are 
connected  and  filled  with  water;  each  cylinder  is  fitted  with 
a  piston;  the  area  of  cross  section  of  the 
small  piston  is  i  sq.  in.,  and  of  the  large 
piston  is  25  sq.  in.  If  the  pistons  are  fric- 
tionless,  and  a  lo-lb.  weight  is  placed  on 
the  small  piston  A,  the  pressure  on  the 
water  in  the  small  cylinder  is  10  Ib.  per  FIG.  21.  — A  weight  of 

,  .  ,.  T»          u      i  10     Ib.     balances     a 

square  inch.     According   to   Pascals   law       weight  of  250  lb. 
this   pressure   is  transmitted   equally  and 
undiminished  by  the  water ;  therefore  the  pressure  upward  on 
the  large  piston  B  is  10  lb.  per  square  inch,  or  a  total  pressure 
of  10  X  25  =  250  lb.    This  is  the  principle  of  the  hydrostatic 
bellows  and  the  hydraulic  press. 


MECHANICS.     LIQUIDS 


The  hydrostatic  bellows.  —  Pascal's  law  can  be  illustrated 
in  a  striking  manner  by  means  of  the  hydrostatic  bellows  shown 
in  Fig.  22.  The  bellows  consists  of  two  disks  of  wood  connected 
by  a  waterproof  canvas  cylinder,  thus  making  a  collapsible 
drum.  A  small  pipe  passes 
through  the  lower  disk  and 
opens  into  the  drum. 

If  now  the  drum  is  filled  with 
water  and  a  man  stands  on  the 
upper  disk,  a  small  amount  of 
water  is  forced  into  the  small 
tube.  The  striking  thing  is 
that  a  very  small  weight  of 
water  will  balance  the  man's 
weight.  In  fact,  a  mere  thread 
of  water  AB  in  the  small  tube 
will  support  any  weight,  pro- 
vided the  disks  are  made  large 
enough. 

Pascal's  law  gives  us  the  ex- 
planation of  this.  If,  for  ex- 
ample, the  small  tube  is  i  sq. 
in.  in  area  and  the  upper  disk 
is  500  sq.  in.  in  area,  i  Ib.  of 
water  in  the  small  tube  above 
the  level  of  the  upper  disk 
exerts  i  Ib.  pressure  on  each 
square  inch  of  the  disk,  or  a 
total  of  500  Ib.  That  is,  i  Ib. 
of  water  in  the  small  tube  supports  500  Ib.  on  the  bellows. 

The  hydraulic  press.  The  hydraulic  press  is  a  device  used 
when  great  pressure  is  required ;  it  is  based  on  Pascal's  law. 
For  example,  if  the  pump  piston  in  A,  Fig.  23,  has  an  area 
of  i  sq.  in.,  and  the  large  piston  in  B  an  area  of  1000  sq.  in., 
and  if  we  neglect  friction,  each  pound  of  pressure  on  the 


FIG.  22.— The  hydrostatic  bellows. 


40 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  23. — The  hydraulic  press. 


pump  piston  exerts  a  total 
force  of  1000  Ib.  on  the  large 
piston. 

Hydraulic  elevators,  hy- 
draulic lift  locks,  hydraulic 
gun  testers,  etc.,  are  based  on 
this  principle. 

Pressure  in  water  pipes. — 
Another  illustration  of  Pas- 
cal's law  is  as  follows.  In 
cities  which  have  neither 
standpipe  nor  reservoir,  the 

water  is  pumped  directly  into  the  mains  and  is  kept  at  a 
certain  pressure.  When  a  fire  occurs  the  pressure  is  increased. 
If  the  pressure  is  increased  20  Ib.  per  square  inch  at  the  pump, 
the  pressure  at  each  hydrant  and  faucet  in  the  city  is  increased 
by  20  Ib.  per  square  inch.  That  is,  the  pressure  exerted  on 
the  confined  liquid  is  transmitted  equally  and  undiminished 
in  all  directions. 

THE  LAW  OF  ARCHIMEDES 

A  body  when  placed  in  a  liquid  loses  weight  equal  to  the  weight 
of  the  liquid  displaced.  This  is  the  law  of  Archimedes. 

This  law  holds  for  bodies  which  sink  in  water  and  for  bodies 
which  float.  We  can  illustrate  it  by  means  of  the  apparatus 
shown  in  Fig.  24,  as  follows : 

Bodies  which  sink  in  water.  — 
Fill  the  large  vessel  with  water 
until  water  runs  out  at  the  spout. 
Weigh  the  small  vessel  empty. 
Weigh  the  heavy  body  in  air 
and  then  when  immersed  in  the 
water  in  the  large  vessel.  Catch 
the  water  displaced  by  the  body,  FlG  24._The  body  loses  weight  equal 

in  the  Small  vessel,  and  weigh  it.       to  the  weight  of  the  liquid  displaced. 


MECHANICS.     LIQUIDS  41 

We  find  that  the  body  loses  weight  equal  to  the  weight  of  the 
water  displaced. 

Bodies  which  float  on  water.  —  If  we  make  the  same  experi- 
ment with  a  floating  body,  we  find  that  the  floating  body  loses 
all  of  its  weight  when  placed  on  the  water  in  the  large  vessel. 
We  find,  however,  that  its  loss  in  weight  is  just  equal  to  the 
weight  of  the  water  it  displaces. 

Cup  and  cylinder. — The  law  of  Archimedes  can  be  illus- 
trated also  by  means  of  the  experiment  shown  in  Fig.  25. 


FIG.    26.  —  The    volume 
of  the  cylinder  is  equal 
FIG.  25.  —  Cup  and  cylinder  experiment.  to  that  of  the  cup. 

The  solid  cylinder  A  is  so  made  that  it  just  fills  the  cup  B. 
That  is,  the  cylinder  has  the  same  volume  as  the  cup  (Fig.  26). 
The  experiment  is  as  follows:  A  is  attached  to  the  bottom 
of  B,  and  both  are  suspended  from  one  pan  of  a  balance.  Weights 
are  added  to  the  other  pan  until  the  cup  and  cylinder  are  just 
balanced.  If  then  a  vessel  of  water  is  raised  up  under  the  cylin- 
der until  it  is  completely  submerged,  the  cup  and  cylinder  lose 
weight,  because  the  water  buoys  up  the  cylinder.  If  now  the 
cup  B  is  filled  with  water,  the  balance  is  restored.  This  shows 
that  the  weight  of  water  which  just  fills  B  is  equal  to  the  weight 
lost  by  A. 


42  PHYSICS  OF  THE  HOUSEHOLD 

Since  A  and  B  have  the  same  volume,  we  see  from  this  experi- 
ment that  when  a  body  is  placed  in  a  liquid  it  is  buoyed  up  by 
a  force  equal  to  the  weight  of  the  liquid  it  displaces.  This  is 
the  law  of  Archimedes. 

Explanation  of  the  law  of  Archimeaes.  —  In  the  experiment 
with  the  lamp  chimney  described  on  page  35  we  learned  that 
water   exerts   pressure   upward,   and    that   this 
upward  pressure  is  equal  to  ^he  pressure  down- 
ward at  the  same  depth. 

In  Fig.  27  a  cube  i  cm.  on  each  edge  is 
represented  as  placed  with  the  lower  surface 
ii  cm.  beneath  the  surface  of  the  water. 


FIG.   27.  —  The       Since  the  area  of  each  surface  is  i  sq.  cm.  and 
buoyant  force  ^  lower  surface  js  XI  cm>  beneath  the  surface, 

is  one  gram. 

the  pressure  upward  on  it  is : 

total  pressure  =aXhXd  =  i  Xn  Xi  =  n  g. 
The  pressure  downward  on  the  top  is: 

total  pressure  =  aX/fXd  =  iXioXi  =  iog. 
The  body  then  is  buoyed  up  with  a  force  ofn  —  io  =  ig. 
Therefore,  it  weighs  i  g.  less  in  water  than  it  does  in  air. 

Since  its  volume  isiXiXi  =  i  c.c.,  it  displaces  i  c.c.  of 
water,  or  i  g.  of  water.  Therefore,  the  loss  in  weight  equals 
the  weight  of  the  liquid  displaced. 

We  see  then  that  the  reason  a  body  loses  weight  in  a  liquid 
is  that  the  upward  pressure  on  the  bottom  is  greater  than  the 
downward  pressure  on  the  top,  by  an  amount  equal  to  the  weight 
of  liquid  displaced  by  the  body. 

APPLICATIONS  OF  THE  LAW  OF  ARCHIMEDES 

Volume  of  irregular  solids.  —  One  application  which  can  be 
made  of  the  law  of  Archimedes  is  in  determining  the  volume 
of  irregular  solids.  For  example,  if  a  body  weighs  200  g.  less 
in  water  than  in  air,  we  know  at  once  from  this  law  that  it 
displaces  200  g.  of  water.  Since  i  g.  of  water  occupies  i  c.c., 


MECHANICS.     LIQUIDS 


43 


the  body  displaces  200  c.c.  of  water;  therefore,  its  volume 
must  be  200  c.c. 

Similarly,  if  we  find  that  a  body  when  weighed  in  water  loses 
a  certain  weight  in  pounds,  we  can  find  its  volume  in  cubic  feet 
by  dividing  the  loss  by  62.5,  the  weight  in  pounds  of  i  cu.  ft.  of 
water.  For  example,  if  the  body  loses  125  Ib.  weight,  its  volume 

is  -^S-  =  2  cu.  ft. 
62.5 

Density  of  a  substance.  —  The  density  of  any  substance  is  the 
weight  of  a  unit  volume  of  that  substance.  Since  in  scientific  work 
the  cubic  centimeter  is  usually  taken  as  the  unit  volume,  the 
density  of  a  substance,  unless  otherwise  stated,  is  understood 
to  be  the  weight  (in  grams)  of  i  c.c.  of  the  substance. 

Density  of  solids  heavier  than  water.  —  The  law  of  Archi- 
medes is  an  aid  to  us  in  finding  the  density  of  solids,  because  it 
gives  us  an  easy 
method  of  find- 
ing the  volume 
of  an  irregular 
solid,  as  ex- 
plained above. 
The  loss  of 
weight  in  grams 
is  numerically 
equal  to  the  vol- 
ume in  cubic 
centimeters. 

To  find  the 
density  of  a  sub- 
stance heavier 

than  water  we  proceed  as  follows :  Find  the  weight  in  grams 
of  the  substance  in  air  and  then  in  water.  Find  the  loss  of 
weight  in  grams ;  this  is  numerically  equal  to  the  volume  in 
cubic  centimeters. 

Divide  the  weight  in  air  by  the  volume  in  cubic  centimeters. 


FIG.  28.  —  Finding  the  density  of  a  body  heavier 
than  water. 


44 


PHYSICS  OF  THE  HOUSEHOLD 


This  gives  the  weight  of  i  c.c.  of  the  substance ;  that  is,  its 
density. 

For  example :  If  the  glass  stopper  shown  in  Fig.  28  weighs 
20  g.  in  air,  and  12  g.  in  water,  the  loss  in  weight  is  8  g. ; 
therefore,  its  volume  is  8  c.c.  We  can  say  then 

8  c.c.  of  the  glass  weighs  20  g. 

.'.  i  c.c.  of  the  glass  weighs  2.5  g. 

.*.  the  density  of  the  glass  is  2.5  g.  per  cubic  centimeter. 

A  short  statement  of  the  work  above  is : 

Density  =  weight  in  air  =  &  =  2.5  g.  per  cubic  centimeter, 
loss  in  water        8 

Density  of  solids  lighter  than  water.  —  Solids  lighter  than 
water  float,  and  in  order  to  find  their  volume  we  must  make 

them  sink.  To  do  this 
we  find  the  weight  of  a 
sinker  in  water  (see 
(i),  Fig.  29)  and  then 
attach  it  to  the  body. 
We  will  illustrate 
the  method  of  finding 
the  density  of  a  float- 
ing body  by  means  of 
an  example.  Let  us 
suppose  that  we  wish 

FIG.  29.  —  Finding  the  density  of  a  body  lighter 

than  water.  to  find  the  density  of 

a  piece  of  cork. 

We  weigh  the  cork  in  air,  the  sinker  in  water,  then  attach 
the  sinker  to  the  cork  and  weigh  them  together  in  water.  Let 
us  suppose  that  the  cork  weighs  10  g.  in  air,  the  sinker  15  g.  in 
water,  and  that  together  they  weigh  5  g.  in  water. 

If  the  cork  and  sinker  were  weighed  as  shown  in  (ii),  Fig.  29, 
they  would  weigh  25  g.,  because  the  cork  in  air  weighs  10  g. 
and  the  sinker  in  water  weighs  15  g. 

When  the  cork  is  placed  with  the  sinker  in  water  as  shown  in 


MECHANICS.     LIQUIDS  45 

(///),  Fig.  29,  they  weigh  only  5  g.    That  is,  the  loss  in  weight 
is  20  g.     Since  this  loss  is  due  to  the  buoyant  force  of  the  water 
on  the  cork,  the  cork  must  displace  20  c.c.  of  water.    Therefore 
the  volume  of  the  cork  is  20  c.c. 
We  can  say,  then : 

20  c.c.  of  cork  weighs  10  g. 

.*.  i  c.c.  of  cork  weighs  f£  =  .5  g. 

The  density  of  cork  is  .5  g.  per  cubic  centimeter. 
If  we  find  it  by  the  short  method  given  above, 

Density  =  weiKht  in  air  =  1°  =  .5  g.  per  cubic  centimeter 
loss  in  water       20 

Density  of  liquids.  —  Three  methods  of  finding  the  density 
of  liquids  are  described  here ;  namely,  by  means  of  the  density 
bottle,  the  density  bulb,  and  the  hydrometer. 

The  density  bottle.  —  The  density  bottle  (Fig. 
30)  is  a  bottle  with  its  volume  marked  on  the 
side,  usually  50  c.c.  or  100  c.c.  The  stopper  of 
the  bottle  has  a  small  hole  bored  through  it  to 
allow  the  surplus  liquid  to  escape. 

In   finding  the  density  of   a   liquid  we   first 
weigh  the  bottle  when  empty  and  then  again 
when  filled  with  the  liquid.     The  difference  in 
weight  is  the  weight  of  50  or   100  c.c.  of  the 
liquid.     The  density  of  the  liquid  is  then  found  Density  botte* 
by   dividing   the   weight   of   the   liquid   by   its 
volume.     For  example,  if  the  bottle  has  a  volume  of .  100  c.c. 
and  the  oil  it  holds  weighs  70  g., 

Density  of  oil  =  -j2^  =  .  7  g.  per  cubic  centimeter 

Density  bulb.  —  The  density  bulb  is  a  bulb  made  of  metal 
or  glass ;  the  method  of  using  it  is  as  follows :  weigh  the  bulb 
in  air  and  then  in  water ;  the  difference  gives  the  volume  of  the 
bulb  in  cubic  centimeters  (law  of  Archimedes). 

Then  weigh  it  in  the  liquid  to  be  tested.    The  difference  be- 


46 


PHYSICS  OF  THE  HOUSEHOLD 


tween  the  weight  in  air  and  in  the  liquid  is  the  weight  of  a 
volume  of  liquid  equal  to  the  volume  of  the  bulb. 

The  density  of  the  liquid  is  then  found  by  dividing  the  weight 
of  the  liquid  by  its  volume. 

Example.  The  bulb  weighs  100  g.  in  air,  60  g.  in  water, 
and  72  g.  in  oil.  The  loss  in  water  is  40  g.,  .*,  the  volume  of 
the  bulb  is  40  c.c.  The  loss  in  oil  is  28  g.,  .'.  40  c.c.  of  the  oil 
weighs  28  g.,  .'.  i  c.c.  of  oil  weighs  f-f  =  .7  g. 

Density  =  .7  g.  per  cubic  centimeter 

Hydrometer. — The  hydrometer,  Fig.  31,  is  made  of  glass 
and  weighted  at  the  bottom  with  shot  or  mercury ;  the  stem  is 
graduated  to  give  the  density  directly.  The 
principle  upon  which  it  works  is  as  follows : 
A  floating  body  sinks  in  a  liquid  until  it 
displaces  its  own  weight  of  the  liquid  (law 
of  Archimedes).  The  hydrometer  sinks  to 
a  certain  mark  in  water.  In  a  liquid  lighter 
than  water,  it  sinks  deeper  before  it  displaces 
its  own  weight  of  the  lighter  liquid.  In 
liquids  heavier  than  water  it  does  not  sink 
so  far  as  it  does  in  water,  because  it  does 
not  need  to  sink  so  far  to  displace  its  own 
weight.  The  stem  is  so  graduated  that  to 
find  the  density  of  a  liquid  it  is  necessary 
only  to  place  the  hydrometer  in  the  liquid 
and  read  the  figure  on  the  stem  opposite 
the  liquid  level.  This  is  the  density  of  the 
liquid.  The  thermometer,  sealed  in  the 
bulb,  gives  the  temperature  at  which  the 
density  of  the  liquid  is  taken. 

Summary.  —  In  this  chapter  we  have 
studied  a  number  of  water  supply  systems  for  cities  and  for 
country  homes.  We  have  also  learned  the  following  laws 
relating  to  liquids : 


FIG.  31.  — The 
hydrometer. 


MECHANICS.     LIQUIDS  47 

(1)  The  law  of  liquid  pressure : 

Pressure  =  area  X  height  X  density 

(2)  Pascal's  law.     Pressure  exerted  on  a  confined  liquid  is 
transmitted  equally  and  undiminished  in  all  directions. 

(3)  The  law  of  Archimedes.     A  body  placed  in  a  liquid  is 
buoyed  up  by  a  force  equal  to  the  weight  of  the  liquid  displaced. 

EXERCISES 

1.  State  the  different  methods  of  supplying  water  to  cities. 

2.  State  the  different  methods  of  supplying  running  water  to  coun- 
try homes. 

3.  Upon  what  does  the  pressure  of  a  liquid  depend? 

4.  What  is  the  hydrostatic  paradox? 

5.  State  Pascal's  law. 

6.  State  the  law  of  Archimedes. 

7.  The  level  of  the  water  in  an  attic  tank  i<*  30  ft.  above  the  level  of 
the  kitchen  faucet.     What  is  the  pressure  per  square  inch  at  the  faucet  ? 

8.  The  level  of  the  water  in  a  city  standpipe  is  80  ft.  above  the 
level  of  a  faucet  in  a  kitchen.     What  is  the  pressure  per  square  inch  at 
the  faucet? 

9.  In  (8)  a  bathroom  faucet  is  15  ft.  above  the  level  of  the  kitchen 
faucet.     What  is  the  pressure  per  square  inch  at  this  faucet? 

10.  The  level  of  the  water  in  a  city  reservoir  is  100  ft.  above  the  city 
mains,  a  kitchen  faucet  is  10  ft.  above  the  mains,  and  a  bathroom  faucet 
is  20  ft.  above  the  mains.  What  is  the  pressure  per  square  inch  at  each 
faucet  ? 

n.  What  height  must  the  water  ki  a  city  reservoir  be  above  the 
kitchen  faucet  to  give  a  pressure  of  40  lb.  per  square  inch  at  the  faucet  ? 
Make  a  sketch. 

12.  In  a  hydraulic  press,  the  area  of  the  small  piston  is  2  sq.  in.,  and 
of  the  large  piston  is  400  sq.  in.     If  100  lb.  force  is  applied  to  the  small 
piston,  what  is  the  upward  lift  on  the  large  piston?     Neglect  friction. 
Make  a  sketch. 

13.  If  in  (12)  a  handle  is  put  on  the  small  piston,  the  force  arm  being 
35  in.  and  the  weight  arm  3^  in.,  what  is  the  upward  lift  on  the  large 
piston  if  the  loo-lb.  force  is  applied  to  the  handle?     Neglect  friction. 
Make  a  sketch. 

14.  A  boy  weighs  126  lb.     When  he  is  entirely  immersed  in  water  he 
weighs  only  i  lb.,  i.e.  i  lb.  of  force  keeps  him  from  sinking  deeper. 
What  is  his  volume  in  cubic  feet? 


48  PHYSICS  OF  THE  HOUSEHOLD 

15.  A  piece  of  rock  loses  24  g.  in  weight  when  placed  in  water.     What 
is  the  volume  in  cubic  centimeters  ? 

16.  A  tank  will  hold  1250  Ib.  of  water.     What  is  its  volume  in  cubic 
feet? 

17.  A  bottle  holds  just  100  g.  of  water  at  4°  C.     What  is  its  volume 
in  cubic  centimeters? 

18.  A  piece  of  rock  weighs  250  g.  in  air,  but  only  150  g.  in  water. 
What  is  its  volume?     What  is  the  density  of  this  rock  in  grams  per 
cubic  centimeter? 

19.  A  piece  of  wood  weighs  180  g.,  a  sinker  weighs  200  g.  in  water. 
When  they  are  weighed  together  in  water  they  weigh  only  20  gr.     What 
is  the  density  of  the  wood  in  grams  per  cubic  centimeter?     Make  a 
sketch. 

20.  A  bottle  holds  100  g.  of  water  at  4°  C.     It  holds  103  g.  of  milk, 
80  g.  of  alcohol,  1 80  g.  of  sulphuric  acid.     What  is  the  density  in  grams 
per  cubic  centimeter  of  milk,  alcohol,  and  sulphuric  acid? 

21.  A  glass  ball  weighs  250  g.  in  air,  150  g.  in  water,  170  g.  in  alcohol. 
What  is  the  density  of  the  alcohol  in  grams  per  cubic  centimeter  ? 

22.  A   ship's    displacement   is    12,500    tons.     What   is    its    weight? 
What  is  the  volume  of  the  hull  below  the  water  line? 

23.  If  your  home  is  supplied  with  running  water,  trace  the  water  pipes, 
starting  at  the  point  where  the  water  enters  the  house  and  following  each 
pipe  to  the  tap  at  which  it  empties.     Make  a  diagram  showing  the  pipes 
from  the  point  at  which  the  water  enters  to  the  principal  fixtures  (sink,  laun- 
dry tub,  bath  tub,  etc.).    At  what  point  could  you  shut  off  the  water  in  case 
of  accident  ? 


CHAPTER  VI 


MECHANICS.     GASES 

THERE  are  many  household  appliances  the  working  of  which 
depends  upon  one  or  more  of  the  physical  properties  of  gases, 
particularly  of  the  gas,  air.  In  this  section  we  shall  study  some 
of  these  appliances,  namely,  different  kinds  of  pumps,  the  pneu- 
matic tank  system  of  water  supply,  the  hydraulic  ram,  the 
vacuum  cleaner,  the  fire  extinguisher,  and  others. 

In  order  to  undersatnd  these  and  similar  appliances,  we  shall 
first  study  the  physical  properties  of  gases  and  the  laws  of  nature 
which  apply  to  gases,  particularly 
to  the  gas,  air. 

Air  has  weight.  — If  we  were  asked 
the  question,  "  How  much  does 
air  weigh?"  we  should  probably 
answer,  "  Air  has  no  weight  at  all." 
A  very  simple  experiment,  however, 
will  show  us  that  air  has  weight, 
and  that  it  would  take  three  or 
four  strong  men  to  lift  a  weight 
equal  to  the  weight  of  air  in  a 
moderately  large  classroom. 

The  experiment  is  as  follows :  A 

glass  flask  (Fig.  32),  fitted  with  a  stopper  and  tap  is  attached 
to  an  air  pump,  and  as  much  as  possible  of  the  air  is  pumped 
out.  The  tap  is  then  closed  and  the  flask  is  attached  to  one 
pan  of  a  scale  and  balanced  by  weights  on  the  other  pan. 
If  now  the  tap  is  opened,  air  enters  the  flask,  and  when  weighed 
E  49 


FIG.  32.  —  Showing  that  air  has 
weight. 


50  PHYSICS  OF  THE  HOUSEHOLD 

the  flask  plus  the  air  in  it  weighs  more  than  the  flask  alone. 
This  shows  that  air  has  weight. 

At  atmospheric  pressure  and  the  ordinary  temperature,  i  cu. 
ft.  of  air  weighs  about  ij  oz.,  and  i  1.  of  air  about  ij  g. 

Weight  of  air  in  a  classroom.  —  Since  we  know  that  i  cu.  ft. 
of  air  weighs  ij  oz.  we  can  calculate  the  weight  of  the  air  in  a 
classroom  of  any  size.  For  example,  let  us  calculate  the  weight 
of  the  air  in  a  classroom  40  ft.  long  by  24  ft.  wide  by  12  ft. 
high.  The  volume  of  the  room  in  cubic  feet  is  40  X  24  X  1 2  = 
11,520  cu.  ft.  Since  each  cubic  foot  of  air  weighs  -f  oz.,  the  air 
in  the  room  weighs  11,520  X  f  =  14,400  oz.,  and  since  there  are 
16  oz.  in  i  lb.,  the  air  weighs  ^r$--  =  9°°  Ik. 

We  see  then  that  it  would  take  a  number  of  strong  men  to 
carry  a  weight  equal  to  the  weight  of  the  air  in  this  classroom. 

ATMOSPHERIC  PRESSURE 

We  have  found  that  air  has  weight  and  we  know  that  we 
all  live  at  the  bottom  of  an  ocean  of  air  (the  atmosphere) 
which  is  some  miles  deep.  It  is  easy  for  us  to  understand 
then  that  this  ocean  of  air  exerts  pressure  on  everything  at 
the  earth's  surface. 

Demonstrations  of  atmospheric  pressure.  —  In  Fig.  33  some  of 
the  methods  of  demonstrating  atmospheric  pressure  are  shown. 
In  I  a  rubber  sheet  is  fastened  air-tight  over  one  end  of  a  hollow 
cylinder,  and  the  other  end  is  connected  with  an  air  pump.  When 
the  air  is  pumped  out,  it  is  found  that  the  rubber  sheet  is  forced 
in  by  the  pressure  of  the  air  above,  see  II. 

In  III  is  represented  a  pair  of  Magdeburg  hemispheres,  which 
were  invented  and  experimented  with  by  Otto  von  Guericke, 
burgomaster  of  Magdeburg,  Germany,  in  1654.  These  hemi- 
spheres are  made  of  iron,  and  the  edges  are  ground  smooth,  so 
that  when  the  two  halves  are  placed  together  the  joint  is  air- 
tight. When  there  is  air  inside,  the  hemispheres  can  be  pulled 
apart  with  ease ;  but  when  the  air  is  pumped  out,  a  large  force 
is  required  to  separate  them.  When  the  hemispheres  are  filled 


MECHANICS.     GASES 


with  air,  the  air  on  the  outside  is  pressing  them  together,  but 
the  air  on  the  inside  is  pressing  them  apart  and  the  one  pressure 
balances  the  other.  When  the  air  inside  is  removed,  however, 


FIG.  33.  —  Illustrating  atmospheric  pressure. 

there  is  left  only  the  air  outside  which  is  pressing  the  hemi- 
spheres together.  In  order  to  separate  them  then  a  force  must 
be  exerted  equal  to  the  pressure  of  the  atmosphere  on  the  outside. 
If  a  tumbler  is  filled  with  water,  then  covered  with  a  piece 
of  paper  and  the  paper  is  held  on  with  the  hand  while  the  glass 
is  inverted,  it  is  found  that  the  paper  remains  on  when  the  hand 
is  removed,  Fig.  34.  _  ^ 

The  reason  is  that  the 
pressure  of  the  atmos- 
phere upward  on  the 
paper  is  greater  than 
the  weight  of  the  water 
in  the  glass. 

If   a    Simp    Can   (Fig.    FIG.  34.  — The  paper 

35)  with  a  little  water 

in  it  is  placed  on  the 

fire  and  the  water  is  allowed  to  boil  for  a  time,  the  steam 

gradually  drives  out  the  air.    If  now  the  can  is  removed  from 

the  fire  and  at  the  same  time  closed  air-tight  with  a  rubber 

stopper,  it  is  found  that  in  a  short  time  the  can  collapses. 


I  n 

FIG.  35.  —  The  can  is 
crushed  by  atmos- 
pheric pressure. 


52  PHYSICS  OF  THE  HOUSEHOLD 

The  reason  for  this  is  as  follows :  When  the  water  has  boiled 
for  some  time,  the  steam  has  driven  out  the  greater  part  of  the 
air,  and  there  is  practically  nothing  in  the  can  then  but  water 
and  steam.  When  the  can  is  closed  air-tight  and  allowed  to 
cool,  the  steam  condenses.  There  is  then  nothing  in  the  can 
but  a  little  water,  the  space  above  the  water  being  nearly  a 
vacuum.  Since  there  is  practically  nothing  inside  pressing  out- 
ward, the  can  must  stand  the  whole  pressure  of  the  atmosphere 
on  the  outside.  If  it  is  not  strong  enough  to  do  this,  it  collapses. 

These  four  experiments  demonstrate  that  the  atmosphere 
exerts  pressure. 

Torricelli.  —  An  Italian  named  Torricelli  (1608-1647)  was 
the  first  to  prove  that  the  atmosphere  exerts  pressure  and  to 
measure  this  pressure.  He  was  led  to  the  discovery  as  follows : 
It  had  been  known  from  ancient  times  that  if  one  end  of  a  pipe 
is  placed  in  water  and  the  air  is  pumped  out  at  the  top,  the  water 
rises  in  the  pipe.  The  ancients  explained  this  by  the  saying, 
"  Nature  abhors  a  vacuum,"  which,  of  course,  was  no  explana- 
tion at  all.  About  1640  a  deep  well  was  dug  near  Florence, 
and  it  was  found  that,  no  matter  how  perfect  the  pump,  water 
could  be  raised  only  33  ft.  It  seemed  then  that  nature's  horror 
of  a  vacuum  stopped  at  33  ft.  The  true  explanation  was  sup- 
plied by  Torricelli.  He  came  to  the  conclusion  that  it  is  the 
pressure  of  the  atmosphere  which  forces  the  water  up  the  pipe, 
and  that  the  pressure  of  the  atmosphere  is  great  enough  to  sup- 
port a  column  of  water  33  ft.  high.  He  reasoned  that,  if  this  be 
true,  a  column  of  any  liquid  heavier  than  water  would  be  raised 
to  a  height  less  than  33  feet.  He  decided  to  make  a  test  with 
mercury,  which  is  13.6  times  as  heavy  as  water,  and  therefore 
would  be  lifted  only  1/13.6  times  as  high  as  water. 

Torricelli's  experiment,  1643.  —  Torricelli's  experiment  is 
as  follows  (Fig.  36) :  A  glass  tube  4  ft.  long,  closed  at  one  end, 
is  entirely  filled  with  mercury.  The  finger  is  then  placed  over 
the  open  end  and  the  tube  is  inverted  over  an  open  dish  of  mer- 
cury. When  the  finger  is  removed  from  the  open  end  under 


MECHANICS.     GASES 


53 


FIG.  36.  —  Torricelli's  ex- 
periment. 


mercury,  the  mercury  in  the  tube  sinks  until  the  level  inside 
is  about  30  in.  above  that  outside. 

Torricelli  concluded  from  this  experiment  that  it  is  the 
pressure  of  the  atmosphere  which  supports  the  column  of  mer- 
cury 30  in.  high,  and  since  mercury 
is  13.6  times  as  heavy  as  water,  the 
atmosphere  should  support  a  column 
of  water  13.6  X  30  =  408  in.  high  = 
34  ft.  high.  It  is  found  by  experi- 
ment that  the  atmosphere  supports 
a  column  of  water  a  little  over  33  ft. 
high.  The  height  is  slightly  less  than 
34  ft.,  because  the  water  evaporates 
into  the  vacuum  at  the  top  of  the 
tube,  and  the  water  vapor  thus  formed 
exerts  a  slight  pressure  downward  on 
the  column  of  water. 

Pascal's  proof,  1648.  —  Pascal  devised  an  experiment  which 
added  further  proof  that  it  is  the  pressure  of  the  atmosphere 
which  supports  the  mercury  in  the  tube.  He  reasoned  that,  if 
it  is  the  atmosphere  which  supports  the  mercury,  then,  since 
as  we  go  up  a  mountain  there  is  less  air  above  us,  the  pressure 
should  be  less  on  a  mountain.  Torricelli's  experiment  was  re- 
peated at  the  base  and  at  the  top  of  a  mountain,  and  it  was  found 
that  the  level  was  lower  by  3  in.  on  the  top  of  a  mountain  two 
thirds  of  a  mile  high. 

The  barometer.  —  The  barometer  is  an  instrument  used  to 
foretell  the  weather.  As  is  seen  in  Fig.  37,  it  is  similar  to  the 
apparatus  used  by  Torricelli  in  his  experiment.  The  pressure 
of  the  atmosphere  on  the  mercury  in  the  short  tube  holds  up 
the  mercury  in  the  long  tube,  and  since  this  pressure  varies  from 
hour  to  hour  the  height  of  the  mercury  varies  also.  Weather 
predictions  are  based  on  this  variation.  The  height  at  sea 
level  is  about  30  in.  or  76  cm.  It  has  been  found  that  if  the 
mercury  falls  much  below  this  height,  stormy  weather  is  to  be 


54 


PHYSICS  OF  THE  HOUSEHOLD 


expected;  and  if  the  mercury  rises  much  above 
it,  we  may  look  for  fine  and  dry  weather. 

Height  of  mercury  independent  of  the  size  and 
shape  of  tube  and  dish.  —  If  we  repeat  Torricelli's 
experiment,  using  a  number  of  tubes  of  different 
sizes  and  shapes,  standing  in  vessels  of  different 
sizes  and  shapes,  we  find,  in  every  case,  that  if 
the  tubes  are  filled  with  mercury  and  then  inverted 
in  a  dish  of  mercury,  as  in  Torricelli's  experiment, 
the  level  is  the  same  in  each  case.  That  is,  the 
height  of  the  mercury  in  the  tube  is  independent 
of  the  size  and  shape  of  the  tube  and  dish,  pro- 
vided the  tube  is  over  30  in.  long  and  the  mercury 
in  the  dish  is  open  to  the  air. 

Pressure  of  the  atmosphere  per  square  inch.  — 
Since  the  height  of  the  mercury  is  the  same,  no 
matter  what  the  size  of  the  tube,  we  may  consider 
the  inside  cross  section  to  be  just  i  sq.  in.  Then 
.it  is  evident  that  the  atmospheric  pressure  on 
i  sq.  in.  at  A,  Fig.  38,  is  holding  up  a  column  of 
mercury  BC  containing  just  30  cu.  in.  of  mercury. 
^ne  cu^c  mc^  °^  mercury  weighs  .49  Ib.  There- 
fore the  pressure  of  the  atmosphere  on  i  sq.  in. 
is  .49  X  30  =  14.7  Ib.  That  is,  the  at- 
mosphere exerts  a  pressure  of  14.7  Ib. 
(nearly  15  Ib.)  per  square  inch,  on  every- 
thing on  the  earth's  surface. 

Pressure  of  the  atmosphere  per  square 
centimeter.  —  To  calculate  the  pressure 
of  the  atmosphere  on  each  square  centi- 
meter, we  consider  the  tube  to  have  an 
inside  cross  section  of  just  i  sq.  cm'. ; 
then  the  atmospheric  pressure  on  i  sq. 
cm.  at  A  holds  up  a  column  of  mercury 
BC  containing  just  76  cu.  cm.  .Now 


1  SQUARE 
INCH  AREA- 


1  SQUAREV 
INCH  AREAj 


FIG.  38.  —  Measuring  at- 
mospheric pressure. 


MECHANICS.     GASES  55 

i  cu.  cm.  of  mercury  weighs  13.6  g.    Therefore  the  pressure 
of  the  atmosphere  on  i  sq.  cm.  is  13.6  X  76  =  1033.6  g. 

LAWS  OF  NATURE  WHICH  APPLY  TO  GASES 

Gases  have  weight  and  exert  pressure.  —  We  have  found 
that  air  has  weight ;  that,  like  liquids,  it  exerts  pressure  on  any- 
thing immersed  in  it ;  and  that  this  pressure  increases  with  the 
depth  of  air. 

In  liquids  the  pressure  is  proportional  to  the  depth  of  liquid 
above  an  object.  This  is  not  true,  however,  in  gases,  because, 
while  all  liquids  are  nearly  incompressible,  all  gases,  as  we  shall 
learn  later,  are  very  compressible,  and  therefore  the  gas  near 
the  bottom  of  a  column  is  denser  than  gas  near  the  top.  For 
example,  a  cubic  foot  of  air  at  the  base  of  a  mountain  has  a 
greater  weight  than  a  cubic  foot  near  the  top. 

Gases,  then,  have  weight,  and  exert  pressure  on  objects  im- 
mersed in  them.  The  other  laws  of  nature  which  we  have 
found  to  apply  to  liquids  also  apply  to  gases,  namely,  Pascal's 
law  and  the  law  of  Archimedes. 

Pascal's  law  applied  to  gases.  —  Pascal's  law  as  applied  to 
gases  is :  pressure  on  a  confined  gas  is  exerted  equally  and  un- 
diminished  in  all  directions.  This  is  shown  in  a  number  of  ways. 

If  in  a  steam  boiler  the  pressure  is  100  Ib.  per  square  inch  on 
one  square  inch  of  the  inside  surface  of  the  boiler,  it  is  found 
to  be  100  Ib.  on  every  other  square  inch  of  the  inside  of  the 
boiler  at  the  same  level.  That  is,  the  pressure  is  transmitted 
equally  and  undiminished  in  all  directions. 

If  a  piece  of  rubber  is  stretched  over  one  end  of  a  lamp  chim- 
ney, and  if  some  of  the  air  is  pumped  out  through  the  other  end, 
the  rubber  is  driven  in  by  the  pressure  of  the  air  on  the  outside. 
If  now  we  move  the  lamp  chimney  around  so  that  the  rubber 
surface  points  upward,  sidewise,  or  downward,  the  rubber 
remains  in  the  same  position.  This  shows  that  the  pressure 
of  the  air  is  equal  in  all  directions  at  the  same  level. 

Also,  when  air  is  pumped  out  of  the  Magdeburg  hemispheres, 


PHYSICS  OF  THE  HOUSEHOLD 


they  are  forced  together  with  the  same  force,  no  matter  how  we 
turn  them,  showing  again  that  the  pressure  of  the  air  is  equal 
in  all  directions  at  the  same  level. 

These  experiments  show  that  Pascal's  law  applies  to  gases. 

The  law   of  Archimedes   applies   to   gases.  —  The   law   of 

Archimedes  as  applied  to  gases  is,  —  All  bodies  immersed  in 

a  gas  are  buoyed  up  by  a  force  equal  to  the  weight  of  the  gas 

displaced.     This  is   shown   by  the   experiment   illustrated   in 

Fig.  39.  A  sealed  glass  globe 
is  balanced  in  air  by  a  small 
brass  or  iron  weight.  Now 
since  the  globe  is  larger  than 
the  weight,  it  displaces  more 
air,  and  therefore  is  buoyed  up 
by  a  greater  force  than  is  the 
weight.  When  the  air  is 
pumped  out  of  the  receiver,  this 
buoyant  force  is  removed  and 
the  balance  is  destroyed. 

The  buoyant  force  on  a  bal- 
loon is  another  illustration  of 
this  law.  The  buoyant  force 
exerted  by  the  air  upon  a  balloon 

is  equal  to  the  weight  of  air  displaced  by  the  balloon.  For 
example,  if  the  balloon  displaces  12,800  cu.  ft.  of  air,  the  total 
buoyant  force  on  it  is  equal  to  the  weight  of  12,800  cu.  ft. 
of  air.  This  is  12,800  X  |  =  16,000  oz.  =  -^y0/-  =  1000  lb. 
The  balloon,  then,  could  lift  a  total  weight  of  1000  lb. 

Laws.     We  have  now  studied  three  different  laws  of  nature 
applying  to  gases,  namely :  — 

1.  Gases  have  weight  and  exert  pressure. 

2.  Pascal's  law. 

3.  The  law  of  Archimedes. 

We  may  now  take  up  two  other  laws,  the  last  which  we  shall 
consider  in  this  chapter,  namely,  Boyle's  law  and  Henry's  law. 


FIG.  39.  —  Illustrating  the  buoyant 
force  of  air. 


MECHANICS.     GASES  57 

Boyle's  law.  —  The  volume  of  a  gas  varies  inversely  as  the 
pressure  on  it.  This  is  an  important  law  which  applies  to  all 
gases.  It  is  called  Boyle's 
law  after  Robert  Boyle, 
an  Englishman,  who  dis- 
covered it  in  1666. 

This  law  means  that  if 
a  gas  is  confined  under  a 
certain  pressure  and  we 
double  the  pressure,  the 
gas  is  compressed  to  one 
half  its  first  volume ;  if  we 
treble  the  pressure,  the  gas 
is  compressed  to  one  third 
its  first  volume;  and  so 
on.  Also  it  means  that  if 
we  halve  the  pressure,  the 
gas  expands  to  twice  its 
first  volume,  and  so  on. 

Boyle's  law  can  be  illus- 
trated by  means  of  the 
apparatus  shown  in  Figs. 
40  and  41. 

The  apparatus  in  Fig.  40 
is  a  U-shaped  tube  with 
mercury  in  the  bend.  The 
short  closed  tube  holds  a 
certain  volume  of  gas,  con- 
fined by  the  mercury.  The 

n  retire  on  tlik  ax*  k  rmf>    FlG'  4<>.  —  Illustrating  Boyle's  law  for  pres- 
sure greater  than  one  atmosphere. 

atmosphere     because     the 

mercury  is  at  the  same  level  in  both  tubes.  If  now  we  pour 
mercury  in  the  large  open  tube,  until  it  stands  30  in.  above 
the  level  in  the  small  tube,  the  pressure  on  the  gas  is  two 
atmospheres  (one  the  air  and  one  the  mercury).  We  find  then 


58  PHYSICS   OF  THE  HOUSEHOLD 

that  the  gas  has  been  compressed  in  the  small  tube  to  one  half 
its  first  volume.  If  we  make  the  pressure  three  atmospheres, 
the  gas  is  compressed  to  one  third  its  first  volume,  and  so  on. 
This  illustrates  Boyle's  law  for  increasing  pressures. 


FIG.  41.  —  Illustrating  Boyle's  law  for  pressures  less  than  one  atmosphere. 

We  can  illustrate  the  law  for  decreasing  pressures  by  means  of 
the  apparatus  shown  in  Fig.  41.  The  large  tube  filled  with 
mercury  holds  a  smaller  tube  which  is  open  at  the  lower  end, 
and  contains  a  small  volume  of  gas. 

When  the  apparatus  is  in  the  position  shown  on  the  right  of 
the  figure,  the  level  of  the  mercury  in  the  tubes  is  the  same ; 
therefore,  the  gas  is  under  a  pressure  of  one  atmosphere.  If 
now  we  raise  the  small  tube  until  the  mercury  in  it  is  15  in. 
(i  atmosphere)  above  that  in  the  large  tube,  the  pressure  on 


MECHANICS.     GASES  59 

the  gas  is  only  \  atmosphere.  We  find  then  that  the  volume 
of  the  gas  is  twice  what  it  was  at  first.  If  we  raise  the  inner 
tube  until  the  mercury  is  20  in.  (f  atmosphere)  above  that  in 
the  large  tube,  the  pressure  on  the  gas  is  \  atmosphere.  We 
find  that  the  volume  of  the  gas  is  three  times  what  it  was  at 
first.  This  illustrates  Boyle's  law  for  decreasing  pressures. 

Henry's  law.  —  If  a  gas  stands  in  contact  with  a  liquid,  part 
of  the  gas  dissolves  in  the  liquid.  This  is  a  general  property 
of  gases.  The  American  scientist,  Henry,  investigated  this 
property  of  gases,  and  in  1803  announced  the  law  which  states 
the  relation  between  the  amount  of  gas  dissolved  and  the 
pressure.  This  is  known  as  Henry's  law.  It  is,  //  a  gas  does 
not  combine  chemically  with  a  liquid,  the  amount  of  gas  dis- 
solved is  directly  proportional  to  the  pressure.  That  is,  if, 
for  example,  we  double  the  pressure,  we  double  the  amount 
dissolved. 

We  have  now  studied  five  laws  of  nature  which  apply  to 
gases : 

1.  Gases  have  weight  and  exert  pressure. 

2.  Pascal's  law. 

3.  Law  of  Archimedes. 

4.  Boyle's  law. 

5.  Henry's  law. 

A  knowledge  of  these  laws  will  enable  us  to  understand  the 
working  of  many  appliances  used  about  the  home  and  elsewhere. 

EXERCISES 

1.  How  can  we  show  that  air  has  weight? 

2.  Describe    four  experiments,  each  of  which  shows  that  the  atmos- 
phere exerts  pressure. 

3.  Describe  Torricelli's. 

4.  How  is  the  pressure  of  the  atmosphere,  per  square  inch,  found? 

5.  State  Pascal's  law  as  applied  to  gases. 

6.  State  the  law  of  Archimedes  as  applied  to  gases. 

7.  State  Boyle's  law. 

8.  Could  you  carry  the  air  contained  in  a  room  40  ft.  long,  20  ft.  wide, 


60  PHYSICS  OF  THE  HOUSEHOLD 

and  10  ft.  high  (i  cu.  ft.  of  air  weighs  i|  oz.)?     What  is  the  weight  of 
the  air  in  the  room? 

9.  How  many  kilograms  of  air  are  there  in  a  room  10  m.  long,  5  m. 
wide,  and  3  m.  high?     (Take  the  weight  of  i  1.  of  air  as  i^  g.) 

10.  A  cubic  inch  of  mercury  weighs  .49  Ib.  What  is  the  pressure  of  the 
atmosphere  on  each  square  inch,  on  days  when  the  barometer  stands 
2Q,  30,  and  31  in.  high? 

11.  A  cubic  centimeter  of  mercury  weighs  13.6  g.     What  is  the  pres- 
sure of  the  atmosphere  on  each  square  centimeter,  on  days  when  the 
barometer  stands  75,  76,  and  77  cm.  high? 

12.  If  the  pressure  of  the  atmosphere  is  14.7  Ib.  per  square  inch,  what 
force  is  necessary  to  pull  apart   Magdeburg  hemispheres  having  an 
inside  diameter  of  4  in.,  when  the  air  is  all  removed  from  the  inside? 

13.  A  rectangular  sirup  can  is  12"  X  6"  X  4".    What  is   the  total 
pressure  of  the  atmosphere  on  the  outside? 

14.  Mercury  is  13.6  times  as  heavy  as  water ;  on  a  day  when  the  mer- 
cury is  30"  high  in  a  mercury  barometer,  how  high  will  the  water  be  in 
a  water  barometer? 


CHAPTER  VII 
AIR  APPLIANCES 

The  lift  pump.  —  The  working  of  the  lift  pump  is  illustrated 
in  Fig.  42.  There  are  two  valves  in  the  pump,  each  of  which 
opens  upward.  One  valve  A  is  located  in  the  plunger  and  the 
other  C  at  the  bottom  of  the  barrel.  When  we  move  the 
plunger  up  and  down  we  remove  air  from  the  barrel  and 


(2)  (3)  (4)  (5) 

FIG.  42.  —  Showing  how  the  lift  pump  works. 

suction  pipe  P.  This  decreases  the  air  pressure  in  the  barrel 
and  suction  pipe  and  the  pressure  of  the  atmosphere,  on  the 
water  in  the  well,  forces  water  up  into  the  pump. 

Since  the  pressure  of  the  atmosphere  is  sufficient  to  lift  water 
only  33  ft.,  the  plunger  is  placed  within  this  distance  of  the 
water.  In  actual  practice  it  is  usually  placed  not  more  than 
20  or  25  ft.  above  the  water.  In  deep  wells  the  plunger  is 
placed  in  a  cylinder,  and  the  cylinder  is  placed  on  the  end  of  a 
pipe  long  enough  to  bring  it  within  20  ft.  of  the  water. 

The  force  pump.  —  When  we  desire  to  force  water  above 
the  level  of  the  pump  spout  we  use  a  force  pump,  one  form  of 

61 


62 


PHYSICS  OF  THE  HOUSEHOLD 


AIR 
CHAMBER 


SUCTION 
-PIPE 


FIG.  43.  — The  force 
pump. 


which  is  shown  in  Fig.  43.  It  is  the  same  as  the  lift  pump 
shown  above  except  that  the  top  of  the  barrel  is  closed.  The 
pump  rod  passes  through  a  stuffing  box  which  is  water-tight. 
The  air  chamber  on  the  discharge  pipe 
serves  to  prevent  strains  on  the  pump, 
and  also  to  keep  up  a  steady  stream 
in  the  pipe.  While  the  plunger  or  lift 
bucket  is  moving  up,  part  of  the  water 
is  forced  into  the  discharge  pipe  and  part 
into  the  air  chamber.  The  water  which 
enters  the  air  chamber  compresses  the  air 
and  increases  its  pressure.  While  the 
plunger  is  moving  down,  the  air  in  the 
chamber  expands  and  forces  the  water 
out  of  the  chamber  and  into  the  discharge 
pipe.  Thus  a  steady  stream  is  maintained 
in  the  discharge  pipe. 
The  pneumatic  tank  system  of  water  supply.  —  The  arrange- 
ment of  a  pneumatic  tank  system  of  water  supply  for  a  private 
home  is  shown  in  Fig.  45.  A  sectional  view  of  the  tank  is 
shown  in  Fig.  44.  The  system 
consists  of  an  air-tight  steel  tank, 
a  force  pump,  and  the  necessary 
pipes.  It  works  as  follows :  Water 
is  forced  into  the  tank  at  the 
bottom.  This  compresses  the  air 
in  the  tank  to  a  smaller  volume 
and  increases  its  pressure.  The 
pressure  of  this  air  forces  water 
up  the  discharge  pipe  to  the  fix- 
tures in  the  rooms  above. 

Boyle's  law,  which  we  studied 
on  page  57,  enables  us  to  understand  a  number  of  things  about 
this  system.  For  example,  if  at  the  beginning  the  tank  is  full  of 
air  at  a  pressure  of  i  atmosphere,  we  know  that  when  the  tank 


DISCHARGE 
PIPE    -- 


— B 
—A 


FIG.  44.  —  Sectional  view  of  the 
pneumatic  tank. 


AIR  APPLIANCES  63 

is  half  full  (A),  Fig.  44,  the  air  is  compressed  to  half  its  volume 
and  the  pressure  is  2  atmospheres;  when  the  tank  is  two 
thirds  full  of  water  (B),  Fig.  44,  the  air  is  compressed  to  one 
third  its  volume,  and  the  pressure  is  3  atmospheres;  when 


FIG.  45.  —  The  pneumatic  tank  system  of  water  supply  for  country  homes. 

the  tank  is  three   fourths  full,  the  air  is  compressed  to  one 
fourth  its  volume,  and  the  pressure  is  4  atmospheres,  and  so  on. 


64 


PHYSICS  OF  THE  HOUSEHOLD 


Lifting  pressure.  —  Since  the  pressure  of  the  atmosphere  is 
exerted  upon  everything,  the  water  flows  from  a  tap  against 
the  pressure  of  i  atmosphere.  For  this  reason,  when  the  actual 
pressure  in  the  tank  is  2  atmospheres,  the  pressure  which  lifts 
the  water,  or  the  lifting  pressure,  is  only  i  atmosphere ;  when 
the  actual  pressure  is  3  atmospheres,  the  lifting  pressure  is  2 
atmospheres;  and  so  on.  A  pressure  of  i  atmosphere  lifts 
water  34  ft.,  a  pressure  of  2  atmospheres  lifts  water  68  ft.,  and 
so  on. 

The  hydraulic  ram.  —  The  hydraulic  ram,  Fig.  46,  is  an 
automatic  pumping  appliance.  It  can  be  used  where  there  is 
a  brook  or  spring  with  a  fall  of  at  least  if  feet.  The  operation 
of  the  ram  is  as  follows.  The  water  from  the  brook  or  spring  flows 


FIG.  46.  —  The  hydraulic  ram. 

down  the  drive  pipe  QP,  and  out  at  the  working  valve  OV. 
The  velocity  of  the  water  rapidly  increases,  and  at  a  certain 
velocity  the  working  valve  V  is  suddenly  closed  by  the  force 
of  the  water.  The  momentum  of  the  water  in  the  drive 
pipe  forces  the  valve  U  open  and  drives  part  of  the  water  into 


AIR  APPLIANCES  65 

the  air  chamber.  The  air  in  the  chamber  is  thereby  com- 
pressed, and  its  pressure  is  increased.  The  air  pressure  stops  the 
water,  and  for  an  instant  starts  it  back  up  the  drive  pipe. 
This  reaction  closes  the  valve  U,  and  allows  the  valve  V  to 
open.  The  water  then  starts  flowing  down  the  drive  pipe  again 
and  out  at  O  V,  this  valve  again  closes  and  more  water  is  forced 
into  the  air  chamber,  and  so  on.  This  operation  is  repeated 
from  20  to  200  times  a  minute,  according  to  the  conditions. 
The  compressed  air  in  the  air  chamber  forces  the  water  out  of 
the  air  chamber  up  through  the  discharge  pipe  S,  and  into  an 
elevated  tank.  From  here  the  water  runs  to  the  house  fixtures 
by  gravity. 

The  air  pump.  —  A  sectional  view  of  an  air  pump  is  shown  in 
Fig.  47.     The  construction  of  the  pump  is  the  same  in  principle 


^  FIG.  47.  —  The  air  pump. 

as  the  ordinary  lift  pump.  One  valve  S  is  in  the  plunger,  and 
the  other  valve  A  is  in  the  base  of  the  pump.  The  rod  at- 
tached to  A  passes  through  the  plunger.  There  is  a  slight 
amount  of  friction  between  the  plunger  and  the  rod,  and  this 
helps  raise  A  on  the  up  stroke  of  the  plunger. 

Air  is  pumped  from  the  receiver  R  as  follows:    When  the 
plunger  moves  up,  the  air  in  the  pump  barrel  KS  is  given  more 
room.     It  expands  and  its  pressure  is  decreased.     The  air  in 
F 


66  PHYSICS  OF  THE  HOUSEHOLD 

R  is  then  at  a  greater  pressure  than  that  in  the  barrel.  It 
therefore  expands  and  part  of  it  passes  through  A  and  into  the 
barrel.  When  the  plunger  moves  down,  this  air  passes  out  of 
the  pump  through  the  valve  S.  On  the  next  upstroke  of  the 
plunger,  part  of  the  air  left  in  R  expands  into  K  H,  and  on  the 
down  stroke  it  is  forced  out.  This  is  repeated  at  each  stroke. 

It  will  be  seen  that  we  pump  air  out  of  anything  by  making 
use  of  one  property  of  gases,  namely,  a  gas  will  expand  indefi- 
nitely if  we  give  it  room  to  expand  (Boyle's  law). 

If,  for  example,  when  the  plunger  moves  up,  it  increases  the 
volume  which  the  air  in  the  receiver  and  cylinder  can  occupy, 
from  say  3  volumes  to  4  volumes,  then  one  fourth  of  the  air 
is  taken  out  in  the  first  stroke.  On  the  second  stroke,  one 
fourth  of  what  is  left  is  taken  out,  on  the  next  stroke,  one  fourth 
of  what  is  left,  and  so  on.  If  the  pump  were  perfect  we  could 
continue  this  indefinitely,  but  we  could  never  get  quite  all  of 
the  air  out  of  the  receiver. 

Vacuum  cleaners.  —  The  vacuum  cleaner  shown  in  Fig.  48 
is  a  hand  power  machine.  Those  shown  in  Figs.  50  and  51 
are  driven  by  means  of  electric  motors. 

In  all  vacuum  cleaners  there  are  three  operations  performed. 
These  operations  are: 

1.  A  partial  vacuum  is  produced  in  the  machine  by  some 
means. 

2.  The  pressure  of  the  atmosphere  forces  air  into  this  partial 
vacuum,  and  as  the  air  enters  through  the  nozzle  or  otherwise 
it  carries  the  dust  and  dirt  in  with  it. 

3.  The  air  is  strained  through  one  or  more  layers  of  cloth 
and  thereby  freed  from  dirt. 

A  sectional  view  of  a  vacuum  cleane  is  given  in  Fig.  49. 
Let  us  see  how  the  three  operations  mentioned  above  are 
performed. 

At  the  beginning  all  the  air  in  the  machine  is  at  the  same 
pressure  as  the  atmosphere  outside.  When  the  handle  of  the 
air  pump  is  moved  back  and  forth,  air  is  pumped  out  of  the 


AIR  APPLIANCES 


67 


cleaning  chamber.  This  decreases  the  pressure  of  the  air,  or, 
in  other  words,  produces  a  partial  vacuum  in  the  chamber. 
This  is  operation  number  one. 

As  soon  as  the  pressure  of  the  air  in  the  cleaning  chamber  is 
less  than  that  of  the  atmosphere,  the  pressure  of  the  atmos- 
phere outside  forces  air  into  the  chamber  through  the  nozzle, 


Cleaning  tool 


FIG.  48.  —  A  hand  power  vacuum  cleaner. 


cleaning  tool,  and  hose,  and  the  dust  and  dirt  are  carried  in 
with  the  air.     This  is  operation  number  two. 

As  the  a.-  passes  through  the  cleaning  chamber  on  its  way  to 
the  pump,  it  is  .strained  through  the  cloth  bag.  It  is  thereby 
freed  from  the  dirt,  which  is  retained  in  the  bag.  This  is  opera- 
tion number  three. 


68 


PHYSICS  OF  THE  HOUSEHOLD 


The  air  pump.  —  The  air  pump  shown  in  Fig.  49  is  what  is 
known  as  a  double-acting  diaphragm  pump.  It  is  in  reality 
two  pumps  in  one.  The  diaphragm  piston  consists  of  a  solid 
disk  in  the  center  of  a  large  disk  of  rubber  or  leather.  This 
piston  is  air  tight.  It  divides  the  pump  into  two  chambers. 


^Cleaning  tool. 


A>r  in  here,. 
FIG.  49.  —  Showing  how  the  vacuum  cleaner  works. 

There  are  two  valves  in  each  chamber,  i  and  3  in  one,  2  and  4  in 
the  other.     All  of  these  valves  open  upward. 

When  the  piston  is  moved  to  the  right,  as  in  the  figure,  the 
size  of  the  left-hand  chamber  is  increased  and  that  of  the  right- 
hand  chamber  decreased.  Air  then  enters  the  left-hand  cham- 
ber through  the  valve  3,  and  the  air  in  the  right-hand  chamber 
is  forced  out  through  the  valve  2.  When  the  piston  is  moved 
to  the  left,  the  air  in  the  left-hand  chamber  is  forced  out  through 


AIR  APPLIANCES 


69 


the  valve  i  and  air  enters  the  right-hand  chamber  through 
the  valve  4.  We  see,  then,  that  on  each  half  stroke  of  the  piston 
air  enters  the  pump  from  the 
cleaning  chamber.  This  shows 
how  the  partial  vacuum  is  pro- 
duced in  the  cleaning  chamber 
by  this  type  of  air  pump. 

The  cleaner  shown  in  Fig.  50 
has  a  diaphragm  pump  driven 
by  an  electric  motor. 

Other  methods  of  producing 
the  partial  vacuum.  —  Vacuum 


Cleaning 
Tool 


leaning 
Chamber 


50.  —  Vacuum  cleaner  with  motor- 
driven  air  pump. 


L/r  Pump 
r^Totor 

_    ^_J__  Jffose 

cleaners  may  be  divided  into 
three   main   classes,   according 
to  the  way  in  which  the  partial  FlG- 
vacuum  is  produced.     It  is  pro- 
duced either  by  an  air  pump,  by  a  fan,  or  by  a  centrifugal  fan. 

The  air  pump 
has  been  de- 
scribed above. 
The  fan  is  similar 
to  the  ordinary 
electric  fan  used 
for  stirring  the 
air  in  a  room. 
The  air  is  driven 
in  a  direction  at 
right  angles  to  the 
plane  in  which 
the  fan  blades  are 
moving.  The 
partial  vacuum 
is  produced  behind  the  fan.  In  the  centrifugal  fan  the  air  is 
driven  towards  the  outer  end  of  the  fan  blades,  and  the  partial 
vacuum  is  produced  at  the  center  of  the  fan. 


~Air Enters  Here. 

FIG.  51.  —  Vacuum  cleaner  with  motor-driven 
centrifugal  fan. 


7o 


PHYSICS  OF  THE  HOUSEHOLD 


The  cleaner  shown  in  Fig.  51  has  a  centrifugal  fan  driven  by 
an  electric  motor.  The  air  enters  just  behind  the  revolving 
brush.  It  passes  into  the  fan  at  the  center  and  is  forced  to  the 
outer  edge  of  the  fan  and  casing.  It  then  passes  through  the 
cleaning  chamber  and  out  into  the  open  air. 

We  see,  then,  that  in  all  vacuum  cleaners  we  make  use  of  one 
of  the  forces  of  nature,  namely,  the  force  of  gravitation.  It  is 
the  attraction  of  the  earth,  or  the  force  of  gravitation,  which  gives 

air  its  weight,  and  which 
thus  produces  the  pressure 
of  the  atmosphere.  All 
vacuum  cleaners  are  so  ar- 
ranged that  the  pressure  of 
the  atmosphere  forces  the 
air  and  dirt  into  the 
chamber. 

The  fire  extinguisher.  — 
The  working  of  the  ordi- 
nary household  fire  extin- 
guisher is  based  upon  the 
physical  and  chemical  prop- 
erties of  the  gas,  carbon 
dioxide. 

The  extinguisher  (Fig.  52) 


FIG.  52. — The  fire  extinguisher. 


is  a  strong  brass  cylinder 
with  a  short  piece  of  stout 

hose  attached  at  the  top.  The  extinguisher  is  charged  as 
follows.  In  the  bottom  is  poured  a  solution  of  i J  Ib.  of  sodium 
bicarbonate  in  2\  gal.  of  water.  Above  this  is  supported  an 
8-oz.  bottle  containing  4  oz.  of  strong  sulphuric  acid.  This 
bottle  is  fitted  with  a  loose  lead  stopper  which  falls  out  when 
the  extinguisher  is  turned  upside  down. 

To  use  the  extinguisher,  it  is  carried  right  side  up  to  the  fire. 
It  is  then  turned  upsidj  dowi  and  the  stream  of  water  and  gas 
is  diluted  upon  the  fire  ty  means  of  the  short  hose. 


AIR  APPLIANCES  71 

The  action  which  takes  place  in  the  extinguisher  is  as  follows : 
When  it  is  turned  upside  down,  the  sulphuric  acid  and  sodium 
bicarbonate  react  chemically  and  produce  a  large  quantity  of 
carbon  dioxide  gas.  The  volume  of  gas  produced  is  many 
times  the  volume  of  the  extinguisher  above  the  solution.  The 
gas  being  confined  in  a  small  volume  has  a  great  pressure 
(Boyle's  law)  and  therefore  drives  the  liquid  out  through  the 
hose  with  great  force.1 

The  fire  is  extinguished,  partly  by  the  water,  and  partly  by 
the  carbon  dioxide  gas.  The  gas  acts  by  smothering  the  fire. 
Let  us  look  into  this  further. 

Carbon  dioxide  has  three  properties  which  make  it  valuable 
in  a  fire  extinguisher :  first,  it  dissolves  readily  in  water,  and  the 
greater  the  pressure,  the  greater  the  amount  of  gas  dissolved 
(Henry's  law) ;  second,  it  does  not  support  combustion ; 
third,  it  is  heavier  than  air. 

When  the  gas  is  produced  in  the  extinguisher  a  large  part  of 
it  is  dissolved  in  the  water  under  the  great  pressure  in  the  ex- 
tinguisher. It  comes  out  of  the  extinguisher  in  the  water. 
Since  the  pressure  of  .the  atmosphere  is  less  than  the  pressure 
in  the  extinguisher,  the  greater  part  of  the  dissolved  carbon 
dioxide  escapes  from  the  water  when  it  leaves  the  extinguisher. 
The  water  moves  from  the  nozzle  to  the  fire  very  rapidly, 
therefore  the  greater  part  of  the  gas  is  released  from  the  water 
after  the  water  is  in  the  fire.  The  released  carbon  dioxide 
surrounds  the  fire  and  displaces  the  air.  It  does  this  the  more 
readily  because  it  is  heavier  than  air.  The  carbon  dioxide  gas 
smothers  the  fire  because  it  does  not  support  combustion  and 
because  it  displaces  the  air,  which  does. 

The  siphon.  —  If  we  fill  a  curved  tube,  AEDB,  Fig.  53,  with 
water,  close  the  ends,  invert  the  tube,  place  the  ends  in  water  and 
open  them,  we  find  that  the  water  runs  uphill  from  A  to  E,  then 
downhill  from  D  to  B.  Let  us  see  why  the  water  runs  uphill. 

1  For  this  reason  the  hose  should  be  grasped  firmly  before  turning  the  cylinder 
upside  down. 


PHYSICS   OF  THE  HOUSEHOLD 


ATMOSPHERIC 
PRESSURE- 


FIG.  53.  — The 
siphon. 


The  atmosphere  presses  upon  the  surface  of  the  water  in 
both  vessels,  A  and  B.     The  pressures  are  practically  equal, 
but  if  anything  a  little  greater  at  B  than  at  A.     In  the  short 
tube   the  atmosphere   holds  up  a  column  of 
water  10  ft.  long ;  in  the  long  tube  a  column 
of  water  15  ft.  long.     Let  us  consider  the  pres- 
sure to  the  right  and  to  the  left  at  the  highest 
point  C. 

The  pressure  to  the  right  is  the  atmosphere 
minus  the  weight  of  10  ft.  of  water.  The  pres- 
sure to  the  left  is  the  atmosphere  minus  the 
weight  of  15  ft.  of  water.  Since  15  is  greater 
than  10,  the  pressure  to  the  right  is  the  greater. 
^[3  Therefore  the  water  at  C  moves  to  the  right, 
the  atmosphere  forces  more  water  up  to  C  from 
A,  it  moves  to  the  right,  and  so  on.  The 
water  thus  moves  from  vessel  A  up  to  C  and 
down  into  the  vessel  B. 

The  trap.  —  The  traps  shown  in  Fig.  54  are  used  on  the  waste 
pipes  from  wash  bowls,  sinks,  bathtubs,  etc.,  to  prevent  sewer 
gas  from  passing  up  into  the  house.  This  is  accomplished  by 
the  water  which  remains  in  the  U-shaped  part  of  the  trap 
after  each  discharge  of  waste  water.  This  is  called  a  water  seal. 
It  will  be  noticed  that  the  part  of  the  trap  to  the  right  of  S 
in  (i)  Fig.  54  is  a  siphon,  and  therefore,  as  the  water  dis- 
charges, the  atmospheric  pressure  forces  the  level  in  tube  T 
down  to  A .  In  the  next  instant  air  is  forced  around  the  bend ; 
atmospheric  pressure  is  restored  in  B  and  the  water  falls  back 
to  the  position  shown  in  (2)  and  (3).  The  middle  pipe  M  in 
modern  traps  is  made  larger  than  the  other  part  of  the  pipe,  as 
shown  in  (3).  With  this  form  of  trap  the  proportion  of  the  water 
carried  over  by  the  siphonage  is  less ;  therefore,  when  the  water 
falls  back  it  has  a  greater  depth,  that  is,  it  makes  a  better  seal. 
The  screw  caps  5  are  placed  at  the  bottom  of  the  traps  in  order 
that  they  may  be  cleaned  out  from  time  to  time. 


AIR    AI'PLIAN 


73 


In  (4)  are  shown  the  air  chambers  used  to  prevent  hammering 
in  the  pipes.  The  hammering  is  the  noise  heard  when  water  is 
shut  off.  It  is  caused  by  the  moving  water  banging  against 
the  end  of  the  pipe  when  it  is  suddenly  brought  to  rest.  The 
air  chamber  is  air  tight  and  full  of  air.  When  the  running 
water  is  shut  off,  it 
moves  up  into  the 
air  chamber,  and  is 
brought  to  rest 
gradually  in  com- 
pressing the  air. 
There  is  no  noise 
because  there  is  no 
sudden  jar  given  to 
the  pipes. 

The  gas  meter.  — 
The  gas  meter  is 
used  to  measure  il- 
luminating gas  as 
it  comes  into  the 
house  from  the 
street  mains.  In 
Fig.  55,  A  is  a  fixed 
partition.  B  and  C 
are  two  movable 
partitions,  each  con- 


Z. 


\Air  Chamber 


FIG.   54.  —  Traps  and  air  chambers. 


nected  to  A  by  four 
plaited  leather  sides, 
making  the  two  chambers  marked  (2)  and  (3)  which  open  and 
close  like  the  sides  of  an  accordion.  The  zigzag  lines  represent 
the  leather  sides.  The  long  arrows  represent  gas  entering  the 
meter ;  the  short  arrows,  gas  going  to  the  burners  and  lights. 
The  gas  entering,  opens  one  leather  chamber  and  closes  the 
other.  In  (a),  (2)  is  being  closed  and  (3)  is  being  opened.  Gas 
is  being  forced  out  of  (2)  and  (4)  and  into  (i)  and  (3).  When 


74 


PHYSICS  OF  THE  HOUSEHOLD 


the  partitions  B  and  C  reach  the  end  of  their  motion  in  one 
direction,  they  force  the  valve  V  in  the  opposite  direction 
(by  a  mechanism  not  shown  in  the  figure)  and  the  gas  enters 
through  the  other  opening,  as  shown  in  (b) .  The  partitions  are 
then  forced  back  by  the  entering  gas  and  (2)  and  (4)  are  rilled 
and  (i)  and  (3)  emptied  into  the  burners  and  lights.  This 


Intake 
P/pe 


FIG.  55. — The  gas  meter. 

operation  is  repeated  over  and  over  again.  The  moving  parti- 
tions move  the  valve  V  and  also,  by  clockwork,  the  hands  on 
the  meter  dial  shown  in  (c).  This  dial  registers  the  number  of 
cubic  feet  of  gas  which  pass  through  the  meter. 

Other  uses.  —  The  properties  of  air  and  gases  are  turned  to 
man's  use  in  many  other  ways.  We  have  not  sufficient  space 
to  describe  them  in  detail,  but  those  who  are  interested  may  find 
descriptions  of  them  in  other  publications.  A  few  of  these 
uses  are  as  follows : 

The  balloon.  —  The  property  of  air  made  use  of  in  the  bal- 
loon is  "  All  bodies  immersed  in  air  are  buoyed  up  by  a  force 


AIR  APPLIANCES  75 

equal  to  the  weight  of  air  displaced  "  (the  law  of  Archimedes). 
The  balloon  bag  is  filled  with  gas  lighter  than  air,  usually 
hydrogen,  which  weighs  about  one  fourteenth  as  much  as  air. 
The  load  which  can  be  carried  is  equal  to  the  weight  of  air 
displaced,  minus  the  weight  of  the  bag,  car,  machinery,  and 
the  gas  in  the  bag. 

Aeroplanes.  —  When  we  move  rapidly  through  the  air,  as 
on  a  bicycle,  automobile,  or  train,  we  feel  the  air  pressing  us 
back,  even  though  it  be  a  perfectly  calm  day.  This  is  because 
air  has  a  certain  mass  (proportional  to  its  weight)  and  offers 
resistance  to  anything  which  shoves  it  aside.  This  is  the 
property  made  use  of  in  aeroplanes.  The  planes  slant  slightly 
downwards  from  front  to  back,  and  as  they  are  moved  rapidly 
through  the  air,  by  the  motors  and  propellers,  part  of  the  force 
of  resistance  exerted  by  the  air  acts  in  a  perpendicular  direction 
and  keeps  the  aeroplane  in  the  air. 

Compressed  air.  —  Compressed  air  is  put  to  many  uses,  for 
example,  in  hammers,  drills,  etc.  In  the  pneumatic  hammer, 
a  piston  is  driven  back  and  forth  by  compressed  air,  very  much 
as  the  piston  of  a  steam  engine  is  driven  by  steam.  This  piston 
strikes  the  blow.  The  advantage  of  the  pneumatic  hammer  is 
that  it  strikes  many  blows  in  the  time  in  which  a  man  could 
strike  one ;  thus  one  man  can  do  the  work  of  many. 

Submarine  divers  are  supplied  with  a  tank  of  compressed  air 
on  their  backs.  This  tank  is  connected  with  the  helmet  and 
suit,  and  the  air  is  used  as  needed.  The  air  that  has  been  used 
is  allowed  to  escape  through  a  small  valve  at  the  base  of  the 
helmet ;  when  the  diver  wishes  to  go  to  the  surface,  he  allows 
his  suit  to  fill  up  with  air ;  thus  he  displaces  more  water  and 
increases  the  buoyant  force  until  it  is  sufficient  to  float  him 
to  the  surface. 

In  many  cases  firemen  use  a  similar  tank  and  helmet  to  enable 
them  to  enter  buildings  filled  with  smoke. 

Diving  bell.  —  If  a  tumbler  is  turned  upside  down  and  forced 
under  water,  it  is  noticed  that  the  water  enters  only  slightly. 


76 


PHYSICS   OF  THE  HOUSEHOLD 


The  action  of  the  diving  bell  (Fig.  56)  is  similar  to  this.  It  is 
an  iron  bell  large  enough  to  hold  one  or  more  men,  heavy  enough 
to  sink  in  water,  and  strong  enough  to  stand  the  pressure  of 
the  water.  It  is  used  to  enable  men  to  work  under  water. 
The  atmospheric  pressure  is  equal  to  the  weight  of  a  column  of 
water  33  ft.  high,  and  therefore  when  the  bell  is  33  ft.  under 
water  the  air  in  it  is  under  a  pressure  of  two  atmospheres  (one 
atmosphere  at  the  surface  and  one  more  when  under  33  ft.  of 
water).  The  air  then  is  compressed  to  half  its  volume.  At 
a  depth  of  66  ft.  the  air  is  compressed  to  one  third  its  volume, 

because  the  pressure  is  3  atmos- 
pheres. At  a  depth  of  66  ft. 
the  water  enters  the  bell  to  two 
thirds  its  height.  Men  cannot 
work  conveniently  in  this  water, 
and  to  get  rid  of  it  compressed 
air  is  pumped  in  through  a  pipe 
until  the  water  is  forced  out  to 
the  bottom  of  the  bell.  Then 
they  work  on  land. 

Air  brakes.  —  Every  railroad 
engine  is  supplied  with  an  air 
compresser  from  which  com- 
pressed air  is  led  through  pipes 
to  the  air  brakes  under  each  car. 

The  air  brakes  are  operated  by  compressed  air,  and  since  the 
engineer  controls  the  compressed  air,  he  controls  all  the  brakes 
on  the  train. 

Pneumatic  tubes.  —  In  many  cities  the  mail  is  distributed  to 
the  sub-postoffices  through  pneumatic  tubes.  These  are 
smooth  tubes  placed  underground.  The  mail  is  placed  in  cylin- 
ders which  just  fit  these  tubes,  and  which  are  driven  through  the 
tubes  by  compressed  air.  In  many  large  stores  money  is  carried 
from  the  counters  to  the  central  office  and  the  change  is  returned 
through  pneumatic  tubes. 


AIR  BUBBLES- ss 


FIG.  56.— The  diving  bell. 


AIR  APPLIANCES  77 

Pintsch  gas  system. — The  gaslight  on  trains  is  an  applica- 
tion of  the  fact  that  gases  are  very  compressible.  Ordinary 
illuminating  gas  is  compressed  under  high  pressure  in  tanks 
under  the  cars.  It  escapes  through  an  automatic  valve  which 
regulates  the  flow,  and  is  carried  in  pipes  to  the  lights  in  the 
cars. 

Summary.  —  In  this  chapter  we  have  studied  the  following 
appliances:  lift  pump,  force  pump,  pneumatic  tank  system 
of  water  supply,  hydraulic  ram,  air  pump,  vacuum  cleaner, 
fire  extinguisher,  siphon,  trap,  air  chamber,  gas  meter,  balloon, 
aeroplane,  pneumatic  hammer,  diving  suit,  diving  bell,  air 
brake,  pneumatic  tube,  and  Pintsch  gas  system. 

EXERCISES 

1.  Describe  what  happens  in  the  first  few  strokes  of  a  lift  pump. 
Make  a  drawing. 

2.  Make  a  drawing  of  a  force  pump,  and  describe  how  it  pumps 
water. 

3.  Make  a  drawing  of  a  pneumatic  tank  system  of  water  supply, 
and  describe  how  it  works. 

4.  Make  a  drawing  of  a  hydraulic  ram  and  describe  how  it  works. 

5.  Make  a  drawing  of  an  air  pump  and  describe  how  it  pumps  air 
out  of  a  flask. 

6.  Make  a  drawing  of  a  vacuum  cleaner,  and  describe  how  it  works. 

7.  What  three  operations  occur  in  all  vacuum  cleaners? 

8.  Describe  how  the  fire  extinquisher  puts  out  a  fire. 

9.  Make  a  drawing  of  a  siphon  and  explain  how  and  why  it  works. 

10.  Make  a  drawing  of  a  trap;  explain  why  it  is  used  and  how  it 
works. 

11.  Make  a  drawing  of  a  gas  meter,  and  explain  how  it  works. 

12.  The  air  in  an  air  chamber  is  at  atmospheric  pressure  (15  Ib.  per 
square  inch)  when  the  pumping  is  started.     What  pressure  does  it  exert 
on  the  water  when  it  has  been  compressed  to  one  third  its  first  volume  ? 

13.  The  air  in  the  air  chamber  of  a  hydraulic  ram  is  at  atmospheric 
pressure  when  the  ram  is  started.     What  pressure  does  the  air  exert  on 
the  water  when  it  has  been  compressed  to  one  fourth  its  first  volume? 

14.  If  the  air  in  an  air  chamber  is  at  a  pressure  of  30  Ib.  per  square 
inch  above  atmospheric  pressure,  how  high  will  it  lift  water? 


78  PHYSICS  OF  THE  HOUSEHOLD 

15.  The  pressure  of  the  air  in  a  pneumatic  tank  is  4  atmospheres. 
How  high  will  it  lift  water  ? 

16.  Unscrew  the  top  of  a  fire  extinguisher  and  examine  the  ulterior. 
Replace  the  top.     Take  the  extinguisher  outside.     Make  a  bonfire.     When 
the  fire  is  burning  vigorously,  grasp  the  hose  firmly,  invert  the  extinguisher, 
and  extinguish  the  fire.     Rinse  the  extinguisher  and  recharge  it  according  to 
the  directions  on  the  case. 

17.  In  your  home  trace  the  gas  pipes  from  the  point  at  which  they  enter 
the  house  to  each  fixture  (if  possible).     Make  a  diagram  showing  the  path 
of  the  gas  from  the  point  it  enters  the  house  to  at  least  two  fixtures.     Read 
the  gas  meter  on  the  same  day  of  the  month  for  a  period  of  six  months  and 
compare  your  readings  with  those  of  the  gas  company. 


CHAPTER  VIII 
HEAT  IN  THE  HOME 

IN  this  chapter  we  take  up  the  study  of  heat  in  Us  relation  to 
the  home.  This  is  probably  the  most  important  part  of  our 
course  in  "  Physics  of  the  Household." 

/'The  common  household  heat  appliances  are  the  grate,  stove, 
(hot-air  furnace,  hot-water  heating  system,  steam  heating  sys- 
tem, double  boiler,  steam  cooker,  fireless  cooker,  thermos  bottle, 
\refrigerator,  and  ice-cream  freezer. 

There  are  many  interesting  questions  which  we  should  like 
to  answer  about  these  and  other  heat  appliances.  In  order  to 
do  so,  however,  it  will  be  necessary  first  to  learn  something  of 
heat  and  of  the  laws  of  nature  which  apply  to  heat. 

The  fire  in  the  home.  —  Next  to  the  members  of  the  family, 
the  fire  is  the  most  important  thing  hi  the  home.  For,  besides 
^  being  the /source  of  heat  and  the  means  of  cooking  food,  it  is 
the  center  of  the  family  life^  It  has  been  so  since  the  begin- 
ning. One  of  the  first  steps  which  distinguished  man  from 
the  lower  animals  was  taken  when  man  learned  to  use  fire,  and 
civilization  has  advanced  with  the  use  of  fire  in  the  home  and  in 
industrial  pursuits. 

At  first  the  fire  was  made  in  the  open,  as  our  camp  fires  are. 
Then  it  was  placed  under  shelter,  and  the  smoke  found  its  way 
out  as  it  could.  Later  a  chimney  was  added,  and  it  became  a 
grate  fire.  In  1742  Franklin  invented  the  first  stove,  and  our 
modern  stoves  and  furnaces  are  later  developments  of  'this. 

For  thousands  of  years  the  social  life  of  our  ancestors  centered 
round  the  fire,  and  the  social  influence  of  the  fire  is  still  very 

79 


8o  PHYSICS  OF  THE  HOUSEHOLD 

strong  in  us.  We  all  like  an  open  fire.  It  seems  to  be  an  in- 
herited instinct;  we  want  to  see  the  fire.  A  group  of  people 
may  be  finding  it  very  hard  to  be  sociable,  but  the  instant  an 
open  fire  is  started,  every  one  is  at  home  and  happy.  A  social 
instinct  is  satisfied. 

The  systems  of  hot-air  heating,  hot-water  heating,  and  steam 
heating,  invented  in  the  last  fifty  years,  are  great  improvements 
over  the  old  method  of  heating  houses.  They  have,  however, 
destroyed  to  some  extent  the  social  influence  of  the  fire.  They 
have  a  scattering  influence  on  the  family,  because  each  member 
of  the  family  can  be  warmed  by  the  radiator  in  his  or  her  room, 
and  there  is  no  one  spot  in  the  home  about  which  the  family 
naturally  gathers.  A  home  which  has  a  modern  heating  sys- 
tem, and  which  also  has  a  family  room  in  which  there  is  an  open 
fire,  retains  the  best  of  both  the  old  and  new  methods  of  heat- 
ing. The  home  is  equably  heated,  and  at  the  same  time  there 
is  one  spot  around  which  the  family  life  centers.  A  grate  fire 
is  an  attractive  open  fire,  but  is  very  inefficient,  the  greater 
part  of  the  heat  being  wasted  up  the  chimney.  An  open 
grate  stove  is  an  improvement,  because  it  is  a  much  more  effi- 
cient heater. 

Let  us  now  take  up  the  study  of  heat,  and  learn  something 
of  the  ways  in  which  man  has  turned  it  to  use  in  the  home. 


f^  EXPANSION 

r 
/  Solids,  liquids,  and  gases  expand  when  heated,  and  contract 

when  cooled.  —  We  may  begin  our  study  of  heat  by  finding  its 
effect  on  solids,  liquids,  and  gases.  This  effect  can  be  illustrated 
by  means  of  the  apparatus  shown  in  Figs.  57,  58,  and  59. 

Solids.  —  The  brass  ball,  Fig.  57,  is  so  made  that  when  it  is 
cold  it  just  passes  through  the  ring.  When  it  is  heated,  however, 
we  find  that  it  does  not  pass  through  the  ring.  This  shows  that 
the  metal  expands  when  heated. 

If  the  ball  is  cooled,  by  placing  it  in  water  or  otherwise,  we  find 


HEAT  IN  THE  HOME 


8l 


that  it  again  passes  through  the  ring.     This 
shows  that  when  metal  is  cooled  it  contracts. 

Liquids.  —  The 
effect  of  heat  on  a 
liquid  can  be  illus- 
trated as  follows: 
Fill  a  flask  with  cold 
water  and  insert  a 
rubber  stopper  and 
glass  tube,  see  Fig. 
58.  Mark  the  level 
of  the  water  in  the  FIG.  58.  — Liquids  ex- 

FIG.    57. — The   brass   ball          pand    when    heated 
expands  when  heated  and 


contracts  when  cooled. 


tube.  If  now  the 
water  is  heated,  the 
level  rises  in  the 
tube,  and  if  it  is  cooled,  the  level 


and    contract 
cooled. 


hen 


FIG.    59.  —  Gases     expand     when 
heated  and  contract  when  cooled. 

G 


falls.  These  experiments 
show  that  the  water  expands  when 
heated,  and  contracts  when  cooled. 

Gases.  —  If  a  flask  filled  with  air 
is  arranged  as  shown  in  Fig.  59, 
and  heated,  the  air  expands,  and 
bubbles  of  air  issue  from  the  lower 
end  of  the  tube.  This  shows  that 
when  air  is  heated  it  expands.  If 
now  the  air  is  allowed  to  cool,  it 
decreases  in  volume,  and  water  is 
forced  up  the  tube  by  atmospheric 
pressure.  This  shows  that  air 
contracts  when  cooled. 

These  experiments  illustrate  the 
effect  of  heat  on  only  one  solid, 
one  liquid,  and  one  gas.  It  is 
found,  however,  that  it  has  a 
similar  effect  on  nearly  all  solids, 
liquids,  and  gases. 


82 


PHYSICS  OF  THE  HOUSEHOLD 


With  a  few  exceptions,  solids,  liquids,  and  gases  expand  when 
heated  and  contract  when  cooled.  This  is  the  law  of  expansion 
and  contraction. 

Rates  of  expansion.  —  It  has  been  found  by  careful  experi- 
ment that^aJLsolids  aBd-alLiiquids  have  different  rates  of  ex- 
pansion, but  that  all  gases  have  the  same  rate  of  expansion?) 
These  properties  can  be  illustrated  by  means  of  the  apparatus 
shown  in  Fig.  60. 

Solids.  —  The  compound  bar,  i,  Fig.  60,  is  made  of  a  bar  of 
brass  riveted  to  a  similar  bar  of  steel.  When  this  compound 
bar  is  heated  it  bends  into  the  form  of  a  curve,  with  the  brass 


/ 


FIG.  60.  —  Solids  and  liquids  expand  at  different  rates,  all  gases  at  the  same  rate. 

on  the  outside  of  the  curve.  This  shows  that  brass  has  a  greater 
rate  of  expansion  than  steel.  It  is  found  by  experiments  of  a 
different  kind  that  brass  expands  about  |  as  much  as  steel  for 
an  equal  change  in  temperature. 

Liquids.  —  One  method  of  comparing  the  rates  of  expansion 
of  liquids  is  illustrated  in  2,  Fig.  60.  In  this  case  the  liquids 
compared  are  water  and  alcohol.  The  flasks  are  of  equal  vol- 
ume and  are  fitted  with  long  tubes  of  equal  size.  One  flask 
is  filled  with  cold  water,  and  the  other  with  alcohol  at  the  same 
temperature.  The  level  of  each  liquid  is  marked  on  the  tube, 
and  the  flasks  are  placed  side  by  side  in  a  vessel  of  warm  water. 
When  the  liquids  have  been  warmed  to  the  temperature  of  the 


HEAT  IN  THE  HOME  83 

warm  water,  it  is  found  that  the  alcohol  has  expanded  much 
more  than  the  water.  This  method  can  be  used  to  compare 
the  rates  of  expansion  of  any  two  liquids. 

Gases.  —  It  is  a  remarkable  fact  that  all  gases  have  the  same 
rate  of  expansion.  This  can  be  illustrated  for  any  two  gases 
by  means  of  the  apparatus  shown  in  3,  Fig.  60.  The  flasks  are 
of  equal  volume,  and  are  fitted  with  tubes  of  equal  size.  The 
lower  ends  of  the  tubes  are  submerged  in  water  to  the  same 
depth,  about  J  in.  In  the  case  illustrated  above,  one  flask 
is  filled  with  air,  and  the  other  with  illuminating  gas.  The 
flasks  are  firmly  attached  to  a  support  (not  shown).  To  warm 
the  gases  to  the  same  temperature,  the  flasks  are  placed  side 
by  side,  and  a  vessel  containing  warm  water  is  raised  under 
them  until  both  flasks  are  submerged  in  the  warm  water. 
When  the  gases  have  ceased  to  bubble  out  of  the  lower  end  of 
the  tubes,  the  vessel  of  warm  water  is  removed  and  the  gases 
are  allowed  to  cool  to  the  temperature  of  the  room.  As  the 
gases  cool  they  contract,  and  the  pressure  of  the  atmosphere 
forces  water  up  the  tubes.  The  heights  of  these  columns  of 
water  serve  as  a  measure  of  the  amount  of  gas  forced  out  of 
each  flask  by  the  expansion.  It  is  found  that  the  water  rises 
to  the  same  height  in  each  tube.  This  shows  that  the  gases, 
air  and  illuminating  gas,  have  the  same  rate  of  expansion. 

Coefficient  of  expansion.  —  The  coefficient  of  expansion  of  any 
substance  is  the  increase  in  unit  length  or  unit  volume  of  the  sub- 
stance when  warmed  i°  C.  For  example,  the  coefficient  of 
expansion  of  iron  is  .000012.  This  means  that  a  bar  of  iron 
one  foot  long  expands  twelve  millionths  of  a  foot  when  warmed 
i°  C.,  or  a  bar  of  iron  i  centimeter  long  expands  twelve  mil- 
lionths of  a  centimeter  when  warmed  i°  C.,  etc. 


84  PHYSICS  OF  THE  HOUSEHOLD 

Coefficients  of  expansion  for  i°  C. 

SOLIDS  LIQUIDS 

Marble 000004  Mercury   .     .     .     .00018 

Pine 000006  Water        .     .     .     .00043 

Glass      .     .     .     .     .     .000009  Petroleum     .     .     .00090 

Platinum 000009  Turpentine    .     .     .00094 

Sandstone oooon  Alcohol     .     .     .     .00120 

Iron        000012  Benzine     .     .     .     .00125 

Copper  .....     .000017 

Brass 000018  GASES 

Aluminium      .     .     .     .000026  All  gases        .     .     .00366 

Lead 000028  =  ^77        of  volume  at  o°C. 

In  the  table  above,  some  solids  and  some  liquids  appear  to 
have  equal  coefficients.  If,  however,  we  give  the  coefficients 
in  greater  detail,  we  find  they  differ  slightly. 

The  laws  of  expansion  and  contraction.  —  We  have  now  illus- 
trated three  laws  of  nature  which  relate  to  heat.  They  are : 

(1)  Nearly  all  substances,  whether  solids,  liquids,  or  gases, 
expand  when  heated  and  contract  when  cooled. 

(2)  All  solids  and  liquids  have  different  coefficients  of  expansion. 

(3)  All  gases  have  the  same  coefficient  of  expansion. 

APPLICATIONS  or  EXPANSION  AND  CONTRACTION 

Before  taking  up  the  other  laws  of  nature  which  relate  to 
heat,  let  us  stop  to  see  how  man  has  made  use  of  his  knowledge 
of  the  laws  of  expansion  and  contraction  in  some  of  the 
ordinary  heat  appliances. 

Thermometers.  —  Thermometers  are  used  to  find  out  how 
hot  or  how  cold  a  body  is.  In  some  cases  the  expansion  and 
contraction  of  solids  is  used  to  indicate  the  temperature.  In 
others,  this  property  of  liquids  or  gases  is  used. 

Solid  Thermometers.  —  A  .  solid  thermometer  is  illustrated 
in  Fig.  61.  The  spiral  AB  is  made  of  two  metals,  a  strip  of 
brass  on  the  outside  and  a  strip  of  steel  on  the  inside.  When 
the  temperature  rises,  the  spiral  becomes  more  curved ;  and  when 
the  temperature  falls,  it  becomes  less  curved.  The  thermometer 


HEAT  IN  THE  HOME 


shown  here  is  a  recording  thermometer.  A  strip  of  paper  is 
fastened  to  the  drum  D,  which  is  revolved  by  clockwork.  The 
point  C  of  the  lever  BC  rests  on  the  paper.  As  the  temperature 
rises,  the  increased  curvature  of  the  spiral  forces  C  up,  and  as 


FIG.  61.  —  A  solid  recording  thermometer. 

the  temperature  falls  the  decreased  curvature  lowers  the  point 
C.  A  pen  attached  to  the  point  C  makes  a  trace  on  the  paper 
and  thus  records  the  temperature. 

The  ordinary  oven  thermometers  are  solid  thermometers. 
In  some  cases  they  are  made  with  a  spiral  of  two  metals,  as 
above.  In  other  cases,  the  expansion  of  a 
metal  bar  is  multiplied  by  levers,  the  last 
lever  being  a  pointer  which  moves  across 
a  temperature  scale. 

Liquid  thermometers.  —  In  ordinary  ther- 
mometers the  expansion  and  contraction  of 
some  liquid,  usually  mercury,  is  used  to 
indicate  the  change  in  temperature.  House- 
hold thermometers  consist  of  a  glass  tube  with  a  bulb  at  one 
end.  The  bulb  and  part  of  the  tube  contain  mercury.  The 
bore  of  the  tube  is  small  compared  to  the  volume  of  the  bulb, 
and  as  a  result,  a  small  expansion  of  the  liquid  in  the  bulb 
produces  a  large  increase  in  the  length  of  the  liquid  column 
in  the  tube.  The  thermometer  scale  is  marked  on  the  tube 
or  on  the  thermometer  back  beside  the  tube. 


FIG.  62.  —  An  oven 
thermometer. 


86 


PHYSICS  OF  THE  HOUSEHOLD 


Gas  thermometers.  —  The    most    accurate   of    all   expansion 

thermometers  1  are  those  which  use  the  expansion  of  a  gas  to 
measure  temperature.  They  are  the 
most  accurate  because  the  rate  of  ex- 
pansion of  gases  is  very  regular.  The 
hydrogen  thermometer  is  the  standard 
thermometer  for  scientific  purposes, 
but  as  it  is  not  a  thermometer  in  com- 
mon use  we  need  not  study  it  here. 

Fixed  points  of  the  thermometer.  — 
In  order  to  mark  the  scale  of  degrees 
on  a  thermometer,  it  is  necessary  to 
have  two  fixed  points,  between  which 
the  divisions  may  be  made. 

It  has  been  found  that  pure  ice 
melts  at  a  constant  temperature,  and 
that  pure  water  boils  r  c 

at  a  constant  tem- 
perature, when  the 
pressure  is  one  at- 
mosphere. These 
two  temperatures 
are  used  as  the  fixed 
points  of  all  ther- 
mometers. 
The  two  thermometers  in  common  use  are 

the  centigrade  and  Fahrenheit  thermometers. 

The  centigrade  thermometer  is  used  in  all 

scientific  work,  and   the   Fahrenheit   is  the 

thermometer  in  common  use  in  Great  Britain 

and  North  America. 

These    thermometers    differ    only   in   the 

manner  in  which  the  scale  is  marked;   see  Fig.  64.     On  the 

centigrade  thermometer  the  melting  point  of  ice  is  called  o°  C., 
1  Electrical  thermometers  are  more  accurate  than  expansion  thermometers. 


"20 


FIG.  63.  —  Household  mercury 
thermometers. 


FIG.  64.  —  Showing 
the  fixed  points  of 
the  Fahrenheit  and 
centigrade  ther- 
mometers. 


HEAT  IN  THE  HOME  87 

and  the  boiling  point  of  water  100°  C.,  and  the  space  between 
these  two  is  divided  into  100  degrees.  On  the  Fahrenheit 
thermometer  the  melting  point  of  ice  is  called  32°  F.,  and  the 
boiling  point  of  water  212°  F.,  and  the  space  between  is  divided 
into  1 80  degrees. 

The  melting  point  of  ice  is  found  by  placing  the  bulb  of  the 
thermometer  in  melting  ice.  The  boiling  point  of  water  is 
found  by  placing  the  bulb  and  stem  of  the  thermometer  in  the 
steam  of  boiling  water. 

Comparison  of  thermometers.  —  From  the  diagrams  of  the 
centigrade  and  Fahrenheit  thermometers,  Fig.  64,  we  see  that 
the  temperature  o°  C.  corresponds  to  32°  F.,  and  100°  C.  to 
212°  F.  Between  o°  C.  and  100°  C.  there  are  100  centigrade 
degrees,  and  between  32°  F.  and  212°  F.  there  are  180  Fahren- 
heit degrees.  We  see  then  that  100  centigrade  degrees  equal 
1 80  Fahrenheit  degrees,  or  i  centigrade  degree  =  -f  Fahren- 
heit degrees.  If  we  have  the  temperature  of  a  body  in 
centigrade  degrees,  and  wish  to  express  it  in  Fahrenheit 
degrees  or  vice  versa,  we  must  remember  that  o°  C.  corresponds 
to  32°  F. 

Example,  (i)  A  temperature  of  20°  C.  is  20  centigrade 
degrees  above  the  melting  point  of  ice ;  this  equals  20  X  f 
=  36  Fahrenheit  degrees  above  the  melting  point  of  ice,  or  a 
temperature  of  32  -f  36  =  68°  F.  That  is,  20°  C.  =  68°  F. 

(2)  A  temperature  of  50°  F.  equals  50  —  32  =  18  Fahren- 
heit degrees  above  the  "melting  point  of  ice;  this  equals  18 
X  f  =  10  centigrade  degrees  above  the  melting  point  of  ice. 
That  is,  50°  F.  =  10°  C. 

The  method  of  transfer  from  degrees  of  one  thermometer 
to  those  of  the  other,  as  illustrated  in  the  examples  above,  can  be 
expressed  as  follows: 

F  =  |  C  +  32,  or 
C  =  |  (F  -  32) 

where  F  and  C  stand  for  temperature  Fahrenheit  and  tem- 
perature centigrade,  respectively. 


88  PHYSICS  OF  THE  HOUSEHOLD 

Applications  of  the  expansion  of  gases  and  liquids.  —  We 

may  now  study  some  other  ways  in  which  man  makes  use  of  the 
expansion  of  gases  and  liquids.  In  doing  so  we  shall  learn  some- 
thing of  the  way  in  which  the  expansion  of  gases  is  used  in 
cooking.  Also  we  shall  answer  such  questions  as 

1.  Why  does  a  stove  draw? 

2.  How  does  a  stove  heat  a  room? 

3.  How  does  a  hot-air  furnace  heat  a  house? 

4.  How  does  a  hot-water  furnace  heat  a  house  ? 

5.  How  is  the  hot- water  system  in  a  house  arranged? 
Expansion    of    gases    in    cooking.  —  Bread.  —  In    making 

homemade  bread,  certain  quantities  of  flour,  water,  salt,  and 
yeast  are  mixed  to  form  a  dough.  The  dough  is  then  set  in  a 
warm  place  to  rise.  The  process  of  rising  is  as  follows:  In 
the  presence  of  moderate  heat  the  yeast  plant  grows  rapidly, 
and  in  the  process  of  growth  gives  off  carbon  dioxide  gas.  This 
gas  is  liberated  all  through  the  dough,  and  forms  thousands  of 
small  closed  pockets  filled  with  the  gas.  As  more  gas  is  liber- 
ated these  pockets  increase  in  size  and  others  are  formed. 
This  expands  the  dough  and  it  rises.  The  dough  is  then 
molded  into  loaves  and  placed  in  the  oven.  The  heat  of  the 
oven  gradually  kills  the  yeast  plant,  and  the  production  of 
carbon  dioxide  gas  ceases.  The  dough,  however,  continues 
to  rise  for  three  reasons :  first,  the  carbon  dioxide  gas  expands 
as  it  is  heated,  and  swells  the  dough ;  second,  the  carbon 
dioxide  gas,  absorbed  by  the  water  at  low  temperatures,  leaves 
the  water  at  high  temperatures  and  swells  the  dough;  third, 
the  water  in  the  dough  turns  to  steam  when  heated,  and  swells 
the  dough. 

In  many  large  modern  bakeries  yeast  is  not  used  in  making 
bread.  The  flour,  water,  and  salt  are  mixed  with  compressed 
carbon  dioxide  gas  in  an  air-tight  mixer ;  here  the  compressed 
carbon  dioxide  gas  is  intimately  mixed  with  the  dough.  When 
the  mixer  is  opened,  the  pressure  on  the  dough  is  decreased, 
the  gas  in  the  dough  expands,  and  the  dough  rises  at  once.  It 


HEAT  IN  THE  HOME  89 

is  then  molded  into  loaves  and  placed  in  the  oven.  The  further 
rising  takes  place  as  described  above.  We  see,  then,  that  the 
expansion  of  gas,  when  heated,  plays  an  important  part  in  the 
making  of  bread. 

Baking  powder. — The  baking  powder  used  in  making  cake, 
biscuit,  etc.,  is  composed  of  substances  which,  in  the  presence 
of  moisture  and  heat,  produce  carbon  dioxide  gas.  This  car- 
bon  dioxide  gas  swells  the  cake  dough  and  makes  it  light.  In 
this  case  the  rising  takes  place  in  the  oven ;  the  heat  of  the  oven 
liberates  and  expands  the  gas,  also  it  turns  the  water  into  steam. 
These  together  cause  the  dough  to  rise. 

Eggs  and  pie  crust.  —  When  an  egg  is  beaten,  air  is  inclosed ; 
similarly,  when  the  dough  for  a  pie  crust  is  rolled  and  folded, 
air  is  inclosed  in  the  dough.  When  the  egg  and  dough  are 
heated,  the  air  in  them  expands  and  they  are  made  lighter. 
It  is  recommended  that  eggs  and  dough,  etc.,  be  mixed  with 
air  in  a  cool  place.  There  are  two  reasons  for  this :  first,  the 
egg  and  dough  are  stronger  and  therefore  hold  more  air;  sec- 
ond, the  cooler  the  air  the  greater  the  change  of  temperature 
when  it  is  heated  to  the  temperature  of  the  oven,  and  therefore 
the  greater  the  expansion. 

Expansion  of  gases  in  cooking.  —  From  the  table  on  page  84, 
we  see  that  the  coefficient  of  expansion  of  all  gases  is  .00366  of 
their  volume  at  o°  C.  for  each  i°  C.  The  Fahrenheit  degrees 
are  only  f  the  length  of  a  centrigrade  degree,  therefore  the 
coefficient  of  expansion  of  gases  is  .00366  X  f  =  .002  of  their 
volume  at  32°  F.  for  each  i°  F.  This  means  that  if  a  gas  is 
inclosed  in  any  material  at  32°  F.  it  will  increase  in  volume 
.002  or  yj^  for  each  degree  Fahrenheit  that  it  is  warmed.  For 
example,  if  air  is  beaten  into  an  egg  at  32°  F.  and  then  warmed 
to  532°  F.  in  the  oven,  the  change  in  temperature  is  532  —  32  = 
500°  F.  The  air  then  expands  |$$,  or  it  just  doubles  in  vol- 
ume. If  it  is  heated  to  432°  F.  it  expands  £££  or  f  >  and  its 
volume  is  f  its  volume  at  32°  F. ;  if  heated  to  332°  F.,  it  ex- 
pands f$#  or  f,  and  its  volume  is  -f  its  volume  at  32°  F. 


9o 


PHYSICS  OF  THE  HOUSEHOLD 


Water  and  air  weigh  less  per  cubic  foot  when  hot  than  when 
cold.  —  Before  trying  to  answer  the  questions,  "  Why  does  a 
stove  draw?  "  etc.,  it  will  be  well  to  show  that  hot  water  weighs 
less  than  the  same  volume  of  cold  water,  and  that  hot  air  weighs 
less  than  the  same  volume  of  cold  air.  This  can  be  shown  by 
experiment,  as  follows : 

To  show  that  hot  water  weighs  less  than  the  same  volume  of 
cold  water.  Weigh  a  flask  filled  to  overflowing  with  cold  water. 
Place  the  flask  in  a  vessel  filled  with  boiling  water.  When  the 
water  in  the  flask  has  ceased  to  expand,  remove  it  from  the  hot 
water,  and  allow  the  outside  to  dry.  Then  weigh  the  flask 
again.  It  will  be  found  that  the  flask  filled  with  hot  water 
weighs  less  than  when  filled  with  cold  water. 

To  show  that  a  flask  filled  with  hot  air  weighs  less  than  when 
filled  with  cold  air.  Fit  a  flask  with  a  rubber  stopper,  and  weigh 
it,  when  filled  with  cold  air,  on  a  delicate  balance.  Remove 

the  stopper  and  heat  the 
flask,  then  replace  the  stop- 
per and  weigh  the  flask  again. 
It  will  be  found  that  the  flask 
when  filled  with  hot  air 
weighs  less  than  when  filled 
with  cold  air. 

Why  does  a  stove  draw  ? 
-To  understand  why  a 
stove  draws  we  will  first 
make  the  experiment  illus- 
trated in  Fig.  65.  The  ap- 
paratus consists  of  a  rec- 
tangular wooden  box  with 
a  glass  front  and  with  two 
holes  in  the  top,  each  fitted  with  a  lamp  chimney.  A  candle 
is  placed  beneath  one  chimney  and  when  it  is  lighted  a  draft 
of  air  passes  down  the  cold  chimney  and  up  the  warm 
chimney.  This  draft  can  be  made  visible  by  holding  lighted 


FIG.  65.  —  A  convection  current  in  air. 


HEAT  IN  THE  HOME  91 

smoke  paper  or  a  lighted  Chinese  joss  stick  over  the  cold 
chimney. 

We  learned  above  that  cold  air  is  heavier,  volume  for  volume, 
than  hot  air.  The  weight  of  any  body  is  due  to  the  attraction 
of  the  earth  for  it.  The  cold  air,  then,  is  pulled  down  by  the  at- 
traction of  the  earth  with  a  greater  force  than  the  hot  air.  The 
cold  air  is  therefore  pulled  to  the  bottom  of  the  box,  and  forces 
up  the  hot  air.  This  cold  air  is  in  turn  warmed  by  the  candle, 
and  is  forced  up  by  more  cold  air.  This  circulation  of  air  con- 
tinues as  long  as  the  candle  burns.  This  is  the  explanation  of 
the  draft  produced  by  the  candle,  and  is  also  the  explanation 
of  the  draft  produced  by  any  fire.  This  movement  of  air  due 
to  difference  in  temperature  is  called  a  convection  current  in  air. 

Draft  in  grate  and  stove.  —  When  a  fire  is  lighted  in  a  grate, 
the  air  in  the  chimney  is  heated.  It  expands  and  becomes 
lighter  per  cubic  foot  than  the  air  in  the  room  and  the  air  out- 
side of  the  house.  The  cooler,  heavier  air  then  sinks  down  on 
account  of  the  greater  attraction  of  the  earth,  and  forces  the 
warm  air  up  the  chimney.  The  cool  air  is  in  turn  warmed  and 
is  forced  up  by  more  cold  air,  see  i,  Fig.  66.  This  process  con- 
tinues as  long  as  the  air  in  the  chimney  is  warmer  than  the  air 
in  the  room.  Similarly,  when  a  fire  is  lighted  in  a  stove,  2,  Fig. 
66,  the  air  in  the  stove,  stovepipe,  and  chimney  is  warmed,  and 
is  forced  up  by  the  cooler  air  from  the  room  and  from  outside 
the  house.  This,  then,  answers  our  first  question,  —  "  Why  does 
a  stove  draw?  " 

We  have  all  observed  that  when  a  fire  is  started  in  a  cold 
stove  the  stove  smokes  for  a  short  time.  The  reason  is  as 
follows:  Before  the  fire  is  lighted,  the  air  in  the  stove,  stove- 
pipe, and  chimney  is  at  the  same  temperature  as  the  air  in  the 
room  and  therefore  there  is  no  draft.  As  soon  as  the  air  in  the 
stove,  stovepipe,  and  chimney  is  heated,  however,  the  cooler, 
heavier  air  of  the  room  is  forced  in  at  every  crevice  in  the  stove 
and  the  smoke  is  carried  up  the  chimney ;  that  is,  the  draft  is 
started. 


PHYSICS  OF  THE  HOUSEHOLD 


When  we  understand  the  cause  of  the  draft  in  a  stove  we  are 
in  a  position  to  understand  the  use  of  the  opening  in  the  stove- 
pipe just  above  the  stove,  3,  Fig.  66.  This  is  opened  to  check 


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i     i 

HI 

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J3J 


D^f^    m  Gr<5te 


Draft  m  Stove ' 


in 
*Stov&  pipe 


3. 


,~  r^>-  -Damper 
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-Stove  pipe 
4: 


FIG.  66.  —  Illustrating  the  draft  in  grates  and  stoves  and  the  use  of  dampers. 

the  draft  in  the  stove.  It  does  this  as  follows :  first,  air  passes 
directly  into  the  stovepipe,  therefore  less  air  passes  through 
the  stove ;  that  is,  there  is  less  draft  through  the  stove :  second, 


HEAT  IN  THE  HOME 


93 


the  cold  air  cools  the  air  in  the  stovepipe  to  some  extent,  there 
is  thus  less  difference  in  the  weight  per  cubic  foot  between  the 
air  in  the  stovepipe  and  that  in  the  room,  therefore  there  is  less 
draft. 

The  stovepipe  damper,  4,  Fig.  66,  checks  the  draft  by  making 
the  passage  in  the  pipe  smaller. 

Since  the  draft  in  a  chimney  is  due  to  the  difference  in  tem- 
perature between  the  air  in  the  chimney  and  that  outside,  it 
can  be  seen  that  a  good  chimney  is  one  that  is  easily  kept  warm. 
For  this  reason  a  chimney  should  be  made  of  non-conducting 
material,  and  should  be  built  inside  the  house  rather  than  out- 
side. A  chimney  built  outside  of  the  house  is  cooled  on  all 
sides,  and  therefore  gives  a  poorer  draft  than  if  it  were  in  the 
house. 

Draft  in  a  kitchen  range.  —  The  arrows  in  Fig.  67  show  the 
path  of  the  draft  in  a  kitchen  range  when  the  oven  is  "  turned 
on."  The  air  enters  below  the  grate,  and  above  the  ash  pan. 
It  passes  up  through  the  fire  in  the  fire  box  and  supplies  oxygen 


AirJEi 


FIG.  67.  — The  draft  in  a  kitchen  range. 


94 


PHYSICS  OF  THE  HOUSEHOLD 


to  the  burning  fuel-.  After  leaving  the  fire  box  it  is  hot  smoke ; 
it  passes  over  the  oven,  under  the  hot-water  reservoir,  and  under 
the  oven.  Beneath  the  oven  there  is  a  partition  which  extends 
about  halfway  from  the  back  to  the  front.  The  hot  smoke 
passes  in  front  of  this,  and  then  up  behind  the  oven  and  into  the 
stovepipe.  We  see,  then,  that  the  hot  smoke  heats  the  oven  on 
all  sides  except  the  front,  where  the  door  is  placed. 

If  the  damper  beneath  the  hot-water  reservoir  is  closed,  the 
hot  smoke  does  not  pass  under  the  hot-water  reservoir.  In 
this  case  the  water  is  not  heated,  but  the  oven  receives 
more  heat,  because  the  smoke  is  hotter  when  it  passes  under 
the  oven. 

If  the  oven  is  not  needed,  a  damper  leading  into  the  stove- 
pipe is  opened  and  the  hot  smoke  passes  directly  from  the  fire 
box  into  the  stovepipe. 

How  does  a  stove  heat  a  room  ?  —  When  a  fire  is  burning  in 
a  stove,  the  air  near  the  stove  is  warmed.  It  therefore  expands 

and  becomes  lighter  per  cubic 
foot  than  the  air  near  the  sides 
of  the  room.  The  cooler,  heavier 
air  then  sinks  down  along  the 
sides  of  the  room,  and  forces 
the  warmer  air  up  to  the  top 
of  the  room,  see  Fig.  68.  This 
cool  air  is  in  turn  warmed,  and 
is  forced  up  by  more  cool  air; 
that  is,  the  air  is  heated  by 
convection.  This  ^convection 
current  continues  as  long  as 
the  stove  is  warmer  than  the  air  in  the  room.  This  explains 
how  a  stove  heats  a  room. 

How  does  a  hot-air  furnace  heat  a  house  ?  —  The  diagram, 
Fig.  69,  illustrates  the  hot-air  furnace.  Examine  this  diagram, 
and  using  your  knowledge  that  warm  air  is  lighter  than  cold  air, 
explain  how  the  hot-air  furnace  heats  a  house. 


FIG.  68.  —  How  a  stove  heats  a  room. 


HEAT  IN  THE  HOME 


95 


FIG.  69. — The  hot-air  heating  system. 


Circulation  of  water.  —  Before  we  study  the  hot-water  heat- 
ing system  and  the  hot-water  tank,  we  may  illustrate  the  circu- 
lation of  water,  due  to  difference  in  temperature,  by  means  of 
the  apparatus  shown  in 
Fig.  70.  It  is  a  rec- 
tangular glass  tube 
filled  with  water.  When 
the  water  in  one  side 
is  heated,  the  water  in 
the  tube  circulates  in 
the  direction  shown  by 
the  arrows. 

The  reason  for  this  is 
as  follows.  Cold  water 
is  heavier  than  the 
same  volume  of  hot 
water.  This  was 
shown  on  page  90. 
The  cold  water,  then,  is  pulled  down  by  the  attraction  of  the 
earth  with  greater  force  than  the  warm  water.  It  is  therefore 
pulled  down  to  the  bottom  of  the  tube,  and  forces  up  the  warm 
water.  This  cold  water  is  in  turn  heated,  and  is  forced  up  by 
more  cold  water.  This  circulation  continues  as  long  as  the 
water  on  one  side  of  the  tube  is  warmer 
than  that  on  the  other.  This  is  the  ex- 
planation of  the  circulation  of  water  in 
the  rectangular  tube,  and  it  is  also  the 
explanation  of  the  circulation  of  water 
in  the  hot-water  heating  system  and 
the  hot-water  tank.  This  movement  of 
water  due  to  difference  in  temperature  is  called  a  convection 
current  in  water. 

The  hot- water  heating  system.  —  In  Fig.  71  is  shown  a  hot- 
water  heating  system.  The  furnace  contains  a  fire  box  and  a 
hot-water  boiler.  The  boiler  is  connected  with  each  hot-water 


FIG.  70.  —  A  convection 
current  in  water. 


PHYSICS  OF  THE  HOUSEHOLD 


radiator  by  two  pipes.  The  system  works  as  follows :  When 
a  fire  is  lighted  in  the  fire  box,  the  water  in  the  boiler  is  heated. 
It  therefore  expands  and  becomes  lighter,  'per  cubic  foot,  than 
cold  water.  The  cooler  water  in  the  radiator,  being  heavier, 
sinks  from  the  radiator  into  the  furnace  boiler  and  forces  the 


FIG.  71.  —  A  hot-water  heating  system. 


hot  water  up  from  the  boiler  into  the  radiator.  This  hot  water 
gives  up  its  heat  to  the  air  in  the  room,  and  thus  cools,  contracts, 
and  becomes  heavier.  It  then  sinks  back  into  the  boiler,  and 
forces  more  hot  water  into  the  radiator.  This  convection 
current  continues  as  long  as  there  is  a  fire  in  the  furnace. 

The  expansion  tank.  —  The  expansion  tank,  Fig.  71,  is  placed 
above  the  highest  radiator.  It  provides  space  for  the  expan- 
sion and  contraction  of  the  water  in  the  system.  The  house- 


HEAT   IN   THE   HOME 


97 


Hot  vsafpr- 


Cold.  WC?^T- 


holder  should  see  that  the  water  level  in  the  tank  is  at  all 
times  above  the  bottom  of  the  water  glass  on  the  tank. 

The  hot-water  tank.  —  The  water  in  a  hot-water  tank  is 
heated  by  convection.  The  manner  in  which  the  tank  is  con- 
nected with  the  water  front  of  a  kitchen  range  is  illustrated  in 
Fig.  72.  The  water  in  the  tank  is  heated  as  follows:  When  a 
fire  is  lighted  in  the 
range,  the  water  in  the 
water  front  is  heated. 
It  expands  and  thus 
becomes  lighter  volume 
for  volume  than  the 
cooler  water  in  the 
tank.  The  cool  water 
then  sinks  from  the 
tank  into  the  water 
front  a/id  forces  the 
hot  water  to  the  top  of 
the  tank.  This  cool 
water  is  in  turn  heated 
and  is  forced  up  by 
cooler  water  from  the 
tank.  By  this  convec- 
tion current  the  water  „. 

TIG.  72. 

is    kept    circulating, 

from  the  bottom  of  the 

tank  to  the  bottom  of  the  water  front,  through  the  water  front 

to  the  top  of  the  tank,  from  the  top  of  the  tank  to  the  bottom, 

etc.    This  explains  how  the  water  in  the  tank  is  heated. 

It  will  be  noticed  in  Fig.  72,  that  the  end  of  the  cold  water 
pipe  is  above  the  level  of  the  water  front.  It  is  arranged  in 
this  way  to  prevent  the  water  from  being  siphoned  out  of  the 
water  front.  If  the  end  of  the  pipe  were  lower  than  the  water 
front,  the  water  might  be  siphoned  out  when  the  water  supply 
was  stopped,  as  follows.  If  a  cold  water  tap  in  the  basement 


he  hot-water  tank  and  the 
water  front. 


98  PHYSICS  OF  THE  HOUSEHOLD 

should  be  open  and  also  a  hot  water  tap  at  any  point  above 
the  basement,  the  cold  water  pipe  would  act  as  a  siphon  and 
lower  the  water  in  the  tank  and  water  front  to  the  level  of 
the  end  of  the  cold  water  pipe  in  the  tank.  If  the  water  front 
should  be  empty  and  there  should  be  a  hot  fire  in  the  range, 
the  water  front  might  be  ruined. 

A  peculiarity  in  the  expansion  of  water.  — When  water  at  o°  C. 
or  32°  F.  is  slowly  warmed,  careful  measurement  of  the  change 
in  volume  discloses  the  remarkable  fact  that  from  o°  C.  to  4°  C. 
the  volume  of  the  water  decreases.  From  4°  C.  upwards  the 

volume  increases.  If  the  measure- 
ments are  made  with  a  Fahrenheit 
thermometer,  the  volume  decreases 
from  32°  F.  to  39.2°  F.,  and  increases 
above  39.2°  F. ;  that  is,  water  is 
heaviest  at  4°  C.  or  39.2°  F. 

The  apparatus  shown  in  Fig.  73 
can  be  used  to  show  that  water  has 
its  greatest  density  at  4°  C.  The 
apparatus  consists  of  a  tall  cylinder 
surrounded  at  the  middle  by  a  cir- 

FIG.  73. — Water  has  its  greatest  J 

density  at  4°  C.  cular  trough.     A  thermometer  is  in- 

serted into  the  cylinder  near  the  top, 

and  another  near  the  bottom.  The  experiment  is  as  follows : 
Fill  the  cylinder  with  water  and  fill  the  trough  with  ice  and  salt. 
Observe  the  change  in  temperature  of  the  water  at  the  top  and 
at  the  bottom  of  the  cylinder.  It  will  be  observed,  after  a  little 
time,  that  the  water  at  the  bottom  is  at  4°  C.,  while  the  water 
at  the  top  is  above  4°  C.  This  shows  that  water  at  4°  C.  is 
heavier  than  water  which  is  warmer.  A  little  later  it  will  be  ob- 
served that  the  water  at  the  bottom  is  still  at  4°  C.,  while  the 
water  at  the  top  is  below  4°  C.  This  shows  that  water  at  4°  C. 
is  heavier  than  water  which  is  colder.  This  experiment  shows 
that  water  at  4°  C.  is  heavier  than  water  above  4°  C.,  and  is  also 
heavier  than  water  below  4°  C.  In  other  words,  it  shows  that 


HEAT  IN  THE   HOME  99 

water  has  its  greatest  density  at  4°  C.  or  39.2°  F.  This  property 
of  water  has  important  consequences  in  nature. 

How  a  lake  cools.  —  A  lake  cools  in  winter  as  follows :  The 
surface  layer  cools,  contracts,  and  sinks ;  the  new  surface  layer 
then  cools,  contracts,  and  sinks.  This  continues  until  all  the 
water  in  the  lake  is  cooled  to  4°  C.  After  this  the  surface  layer 
cools  below  4°  C.,  expands,  and  remains  at  the  surface.  After 
cooling  to  o°  C.  the  surface  layer  turns  to  ice,  and  in  doing  so 
expands  still  further. 

Since  all  the  water  in  the  lake  cools  to  4°  C.  before  any  freezing 
takes  placCj  a  deep  lake  freezes  more  slowly  than  a  shallow  lake 
of  the  same  size.  Since  water  at  4°  C.  is  heavier  than  water 
below  this  temperature,  the  water  at  the  bottom  of  a  lake  is  at 
4°  C.  in  the  coldest  weather ;  thus  aquatic  plants  and  animals 
are  not  destroyed. 

NATURE  OF  HEAT 

Heat  a  form  of  energy.  —  Until  about  a  hundred  years  ago 
heat  was  believed  to  be  a  weightless  fluid.  This  supposed  fluid 
was  called  caloric.  It  was  believed  that  when  a  substance  was 
warmed  by  a  fire,  caloric  flowed  from  the  fire  into  the  substance ; 
and  when  the  substance  cooled,  caloric  flowed  out  of  the  sub- 
stance into  the  air.  This  theory  accounted  for  many  of  the  ob- 
served effects  produced  by  heat,  but  it  failed  to  account  for  the 
heat  produced  by  friction,  and  for  this  reason  it  was  discarded. 

It  is  now  known  that  heat  is  a  form  of  energy.  This  needs 
further  explanation. 

The  molecular  constitution  of  matter.  —  For  very  many  reasons, 
some  of  which  will  be  given  later,  scientists  believe  that  all  sub- 
stances are  composed  of  very  minute  particles  which  are  called 
molecules.  The  molecule  of  any  substance  is  defined  as  the 
smallest  particle  of  that  substance  which  can  exist,  and  still 
retain  the  properties  of  the  substance.  For  example,  a  mole- 
cule of  salt  (sodium  chloride)  is  the  smallest  particle  of  salt 
that  can  exist  and  still  retain  the  properties  of  salt.  It  is  be- 


100  PHYSICS  OF  THE  HOUSEHOLD 

lieved  that  the  molecules  exert  an  attractive  force  upon  one 
another,  and  that  it  is  this  force  which  holds  the  molecules  to- 
gether. This  force  is  called  cohesion. 

Heat  is  the  energy  of  molecular  motion.  —  It  is  known  from 
experiment  that  heat  is  a  form  of  energy  because  it  can  be  pro- 
duced from  and  converted  into  other  forms  of  energy,  such  as 
mechanical  energy,  chemical  energy,  electrical  energy,  etc. 
Heat  is  believed  to  be  the  energy  of  motion  of  the  molecules. 
According  to  this  theory,  when  a  body  is  absolutely  cold,  that 
is,  when  it  is  at  a  temperature  of  —273°  C.,  the  molecules  are 
still  and  are  held  in  contact  with  one  another  by  the  force  of 
cohesion.  When  a  body  is  warmed,  however,  the  molecules 
are  set  in  motion;  they  move  about  and  strike  against  each 
other ;  and  the  force  of  these  collisions  separates  them  slightly 
against  the  force  of  cohesion.  The  higher  the  temperature,  the 
greater  the  velocity  of  the  molecules,  and  therefore  the  farther 
they  are  separated  against  the  force  of  cohesion. 

Why  do  substances  expand  when  heated  and  contract  when 
cooled  ?  The  theory  that  heat  is  "  the  energy  of  molecular 
motion  "  accounts  for  the  expansion  of  a  body  when  heated. 
When  the  body  is  warmed  the  molecules  move  more  rapidly 
and  strike  each  other  with  greater  force ;  therefore,  they  separate 
farther  against  the  force  of  cohesion.  If  each  molecule  is  a 
little  farther  from  its  neighbors,  the  body  as  a  whole  occupies 
more  space,  that  is,  it  expands. 

This  theory  also  explains  why  a  body  contracts  when  cooled. 
When  the  body  cools,  its  molecules  move  with  less  velocity  and 
strike  each  other  with  less  force ;  therefore,  the  force  of  cohesion 
draws  each  molecule  closer  to  its  neighbors.  The  body  as  a 
whole  then  occupies  less  space,  that  is,  it  contracts. 

Summary.  —  In  this  chapter  we  have  studied:  expansion  and 
contraction,  thermometers,  the  expansion  of  gases  in  cooking, 
the  draft  of  a  stove,  how  a  stove  heats  a  room,  the  hot-air  heat- 
ing system,  the  hot- water  heating  system,  the  hot  water  tank, 
how  a  lake  cools,  and  the  nature  of  heat. 


HEAT  IN  THE   HOME      .«,    ,  .,  j,oi. 


EXERCISES 

1.  How  can  we  show  that  solids,  liquids,  and  gases  expand?     Make 
drawings. 

2.  How  can  we  show  that  solids  and  liquids  have  different  rates  of 
expansion,  and  that  gases  have  the  same  rate?     Make  drawings. 

3.  Describe  a  solid  thermometer. 

4.  Draw  a  Fahrenheit  and  a  centigrade  thermometer,  and  mark  the 
freezing  and  boiling  point  of  water  on  each. 

5.  How  many  Fahrenheit  degrees  are  there  between  the  freezing 
and  boiling  points  of  water?     How  many  centigrade  degrees? 

6.  Show  that  i  Fahrenheit  degree  is  equal  to  f.  centigrade  degrees, 
and  that  i  centigrade  degree  is  equal  to  |  Fahrenheit  degrees. 

7.  Show  that  o°  C.,  20°  C.,  and  50°  C.  are  the  same  as  32°  F.,  68°  F., 
and  122°  F. 

8.  What  centigrade  temperatures  correspond  to  40°  F.,  86°  F.,  158°  F.? 

9.  Explain  how  homemade  bread  dough  is  made  lighter. 
10.  Explain  how  baker's  bread  is  made  lighter. 

n.  Explain  how  cake  and  biscuit  dough  are  made  lighter. 

12.  Why  is  it  advisable  to  beat  eggs  in  a  cool  place? 

13.  Air  is  folded  into  the  dough  of  pie  crust  at  32°  F.     The  pie  is 
baked  in  an  oven  at  332°  F.     How  much  does  the  air  expand? 

14.  Why  does  a  stove  draw?     Make  a  drawing. 

15.  Make  a  drawing  showing  the  path  of  the  draft  through  a  kitchen 
range.     Describe  it. 

16.  How  does  a  stove  heat  a  room?     Make  a  drawing. 

17.  How  does  a  hot-air  furnace  heat  a  house?     Make  a  drawing. 

18.  How  does  a  hot-water  furnace  heat  a  house?     Make  a  drawing. 

19.  How  is  the  water  in  a  hot- water  tank  kept  hot?     Make  a  drawing. 

20.  Describe  three  or  more  other  ways  in  which  man  has  made  use 
of  the  laws  of  expansion. 

21.  What  is  heat? 

22.  State  the  molecular  theory  of  matter. 

23.  What  is  cohesion  ? 

24.  Why   do   substances   expand   when   heated   and   contract  when 
cooled  ? 

25.  In  your  home  examine  the  appliance  by  which  water  is  heated  for  the 
sink  and  bath.     Trace  the  path  of  the  water  from  the  cold  water  pipe 
through  the  hot  water  tank,  the  water  front,  and  to  the  taps  at  the  sink 
Make  a  diagram  showing  the  path  of  the  water. 


CHAPTER  IX 
MOVEMENT  OF  HEAT.    HEAT  APPLIANCES. 

WE  all  know  of  many  cases  in  which  heat  moves  from  one 
place  to  another;  for  example,  from  a  fire  to  the  food  being 
cooked,  from  a  furnace  to  the  rooms  heated,  and  from  the  sun  to 
the  earth.  We  also  know  of  cases  in  which  the  movement  of 
heat  is  prevented  or  decreased ;  for  example,  by  the  clothes  we 
wear,  by  the  walls  of  houses,  by  the  sawdust  in  ice  houses,  and 
by  double  windows.  In  this  section  we  will  study  first  the  differ- 
ent ways  in  which  heat  moves  from  one  place  to  another,  and 
then  we  will  study  a  number  of  household  appliances  which  man 
has  devised  to  control  the  movement  of  heat. 

There  are  three  ways  in  which  heat  moves  from  one  place  to 
another,  namely,  by  conduction,  by  convection,  and  by  radia- 
tion. 

Conduction.  —  If  a  rod  of  metal  is  held  with  one  end  in  a 
flame,  it  is  found  that  the  heat  moves  along  the  metal  readily 
and  that  in  time  the  metal  becomes  too  hot  to  hold.  If  a  rod 
of  glass  is  held  in  the  flame  in  a  similar  manner,  however,  it  is 
found  that  the  heat  moves  along  the  glass  very  slowly.  These 
experiments  show  that  the  metal  is  a  good  conductor  of  heat 
and  that  glass  is  a  poor  conductor  of  heat.  When  heat  moves  from 
one  part  of  a  substance  to  another  without  visible  motion  of  the  parts 
of  the  substance,  the  method  of  heat  movement  is  known  as  conduc- 
tion. The  process  of  conduction  is  as  follows :  The  molecules 
of  that  part  of  the  body  which  is  heated  vibrate  very  rapidly. 
These  molecules  strike  the  molecules  near  them,  and  set  them  in 
more  rapid  vibration.  These  molecules  strike  others  farther 

102 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES 


103 


FIG.  74. —  Conduction  in  solids. 


away,  and  so  on.  Thus  the  energy  of  motion  of  the  molecules, 
or  heat,  ntoves  from  the  hot  part  of  the  body  to  the  cooler  part. 
We  can  make  a  rough  comparison  of  the  conducting  power  of 
solids  by  the  experiment  illustrated 
in  Fig.  74.  Rods  of  the  solids  are 
coated  with  paraffin  and  placed  on 
a  non-conducting  block,  with  one 
end  of  each  rod  in  a  flame.  The 
paraffin  is  melted  most  rapidly  on 
the  solid  which  has  the  greatest 
conductivity. 

Conduction  in  liquids  and  gases. 
—  All  liquids,  except  liquid  metals, 
are  very  poor  conductors  of  heat. 
This  can  be  shown  by  the  experi- 
ments illustrated  in  Fig.  75.  If  a 
test  tube  filled  with  water  is  held 
in  the  hand,  and  the  water  at  the  top  of  the  test  tube  is  heated, 
the  water  can  be  made  to  boil  at  the  top  while  the  water  at 
the  bottom  remains  cold.  The  molecules  of  water  at  the  top 
are  in  very  rapid  vibration,  but  the  energy  of  vibration  does 

not  move  readily  from  one  part  of 
the  liquid  to  the  other.  That  is, 
water  is  a  poor  conductor  of  heat. 

It  is  very  difficult  to  illustrate  the 
conducting  power  of  gases,  on  account 
of  the  disturbing  effects  of  convection 
and  radiation.  The  results  of  the 
experiments  which  have  been  made, 
however,  show  that  gases  are  very 
poor  conductors  of  heat.  This  is  shown 
in  many  familiar  ways ;  double  doors 

and  double  windows  are  effective  in  retarding  the  escape  of 
heat,  because  the  air  between  the  doors  and  between  the  win- 
dows is  a  poor  conductor  of  heat.  Also,  fur,  feathers,  straw, 


FIG.  75.  —  Liquids  are  poor 
conductors  of  heat. 


104  PHYSICS  OF  THE  HOUSEHOLD 

etc.,  are  poor  conductors  of  heat,  chiefly  because  they  contain 
a  great  deal  of  air. 

Convection.  —  Liquids  and  gases  are  poor  conductors  of  heat, 
but  they  transfer  heat  readily  from  one  place  to  another  by  the 
process  known  as  convection.  The  movement  of  heat  from  one 
place  to  another  by  the  movement  of  the  heated  matter  is  known  as 
convection.  We  have  already  illustrated  convection  in  gases 
and  liquids  by  means  of  the  apparatus  shown  in  Fig.  65  and 
Fig.  70.  When  a  gas  or  liquid  is  heated,  it  expands  and  there- 
fore becomes  lighter,  volume  for  volume,  than  the  cold  gas 
or  liquid.  The  cool  gas  or  liquid  then  sinks  down  and  forces 
the  warm  gas  or  liquid  to  a  higher  level.  This  is  the  cause  of 
convection.  The  currents  of  air  or  liquid  thus  set  up  are 
called  convection  currents. 

In  the  last  chapter  we  studied  a  number  of  ways  in  which  man 
makes  use  of  convection  in  air :  in  the  draft  of  a  stove ;  in  the 
hot-air  furnace ;  etc. ;  also  ways  in  which  he  makes  use  of  con- 
vection in  liquids ;  in  the  hot- water  heating  system,  in  the  hot- 
water  tank,  etc. 

Radiation.  —  A  person  sitting  in  front  of  a  grate  fire  receives 
heat  from  the  fire  by  radiation.  A  person  standing  in  sunlight 
receives  heat  from  the  sun  by  radiation.  A  body  held  near 
an  incandescent  electric  light  receives  heat  from  the  filament 
by  radiation.  These  are  examples  of  the  movement  of  heat 
by  radiation.  The  process  of  heat  movement  by  radiation  is  as 
follows :  The  molecules  of  the  hot  body  are  in  very  rapid 
vibration ;  each  to  and  fro  vibration  of  a  molecule  sets  up 
a  wave  in  the  ether;  when  these  waves  fall  upon  a  body, 
they  set  the  molecules  of  the  body  in  more  rapid  vibration, 
that  is,  they  heat  the  body.  The  movement  of  heat  from 
one  place  to  another  by  means  of  ether  waves  is  known  as 
radiation. 

The  ether  is  believed  to  be  a  medium  which  fills  all  space,  the 
space  between  the  planets,  and  the  space  between  the  molecules 
and  atoms  of  bodies.  It  has  been  necessary  to  assume  the  ex- 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES  105 

istence  of  such  a  medium  in  order  to  account  for  many  heat, 
light,  and  electrical  phenomena. 

The  waves  set  up  in  the  ether  by  the  molecules  of  a  hot  body 
constitute  a  form  of  energy  called  radiant  energy.  These  waves 
are  similar  to  light  waves  and  will  be  discussed  further  in  the 
chapters  on  Light.  It  may  be  said  here,  however,  that  the  waves 
of  radiant  energy  travel  in  straight  lines,  with  a  velocity  of 
186,000  miles  a  second,  and  may  pass  through  a  medium  without 
heating  it.  Radiant  energy  is  not  heat,  but  it  is  converted  into 
heat  when  it  falls  upon  a  substance  which  absorbs  it,  and  for 
this  reason  these  waves  are  commonly  called  heat  waves. 

We  can  obtain  a  clearer  idea  of  radiation  and  of  the  distinc- 
tion between  radiation,  conduction,  and  convection  by  consider- 
ing further  a  number  of  familiar  examples  of  radiation. 

Radiation  from  a  grate  fire.  A  person  sitting  in  front  of  a 
grate  fire  does  not  receive  heat  by  conduction  because  air  is  a 
very  poor  conductor  of  heat.  He  does  not  receive  heat  by  con- 
vection, because  the  convection  currents  move  towards  the  fire 
and  up  the  chimney.  He  receives  heat  only  by  radiation.  The 
molecules  of  the  burning  fuel  are  in  rapid  vibration,  and  set  up 
waves  in  the  ether.  When  these  waves  fall  upon  any  body,  they 
set  the  molecules  of  the  body  in  more  rapid  vibration,  that  is, 
they  heat  the  body. 

Radiation  from  the  sun.  —  Heat  passes  from  the  sun  to  the 
earth  by  radiation.  The  molecules  of  the  substances  in  the  sun 
are  in  very  rapid  vibration  and  set  up  waves  in  the  ether.  These 
waves  travel  in  all  directions  from  the  sun,  and  a  small  part 
of  them  fall  upon  the  earth.  When  they  fall  upon  any  body  on 
the  earth,  they  set  the  molecules  of  the  body  in  more  rapid  vi- 
bration, that  is,  they  heat  the  body. 

Radiation  from  an  incandescent  electric  light.  •-  When  the 
hand  is  held  near  a  lighted  incandescent  electric  light,  it  receives 
heat  from  the  hot  filament  by  radiation.  It  does  not  receive 
heat  by  conduction  because  the  interior  of  the  bulb  is  a  vacuum 
and  the  glass  is  a  very  poor  conductor  of  heat.  It. .-does. not 


106  PHYSICS  OF  THE  HOUSEHOLD 

receive  heat  by  convection,  because  there  is  no  gas  in  the  bulb  to 
carry  the  heat  by  convection.  The  movement  of  heat  by  radia- 
tion is  specially  interesting  in  this  case  because  it  shows  that 
the  waves  of  idiant  energy  pass  through  a  vacuum  as  readily 
as  they  do  through  air,  glass,  etc.  As  a  matter  of  fact  it  can  be 
shown  by  experiment  that  radiant  energy  passes  through  a 
vacuum  more  readily  than  it  does  through  any  substance. 

APPLIANCES  WHICH  CONTROL  THE  MOVEMENT  OF  HEAT 

We  have  learned  above  the  three  ways  in  which  heat 
moves  from  one  place  to  another,  namely,  by  conduction,  by 
convection,  and  by  radiation.  We  will  now  study  a  number 
of  appliances  which  man  has  devised  to  control  the  movement 
of  heat. 

Specific  thermal  conductivity.  —  The  specific  thermal  conduc- 
tivity of  any  material  is  the  number  of  calories  of  heat  which  pass 
per  second  across  each  square  centimeter  of  a  layer  of  the  material 
i  cm.  thick,  when  the  difference  in  temperature  between  the  sur- 
faces of  the  layer  is  i°  C. 

Specific  Thermal  Conductivity  of  Materials 

Silver    .     .     .  i.o  Glass 002  Silk       .     .     .  .00022 

^Copper ...      .9  Water      .     .     .     .0014  Mineral  wool .  .00019 

Gold     ...      .7  Cement    .     .     .     .0007  Cork     .     .     .  .00013 

y*\luminium     .       .5  Asbestos  paper .     .0006  Sawdust    .     .  .00012 

Brass    ...      .26  Cotton  cloth     .     .00055  Felt  ....  .00009 

Tin 16  Wood 0005  Horn     .     .     .  .000085 

^Eron       ...       .12  Paper 0003  Cottonwool   .  .00005 

Porcelain  .     .      .0025  Flannel    .     .     .     .00023  Air 00004 

From  the  table  above  we  see  that  metals  are  good  conductors 
of  heat,  and  that  nonmetals  are  poor  conductors  of  heat.  We 
see  also  that  water  is  a  poor  conductor  and  that  air  is  a  very  poor 
conductor  of  heat. 

If  three  cooking  utensils,  made  of  copper,  aluminium,  and 
iron  respectively,  are  of  the  same  size  and  of  the  same  thickness, 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES 


107 


we  can  see  from  the  table  that  the  copper  utensil  will  conduct 
over  seven  times  as  much  heat,  and  the  aluminium  utensil  over 
four  times  as  much  heat  as  the  iron  utensil,  in  the  same  time, 
and  from  the  same  fire.  Tinware  is  sheet  iron  covered  with 
a  thin  coating  of  tin ;  tinware  utensils,  therefore,  may  be 
considered  as  iron  utensils. 

Cooking  utensils.  —  Cooking  utensils  are  made  of  metal  for  a 
number  of  reasons :  first,  the  metals  do  not  bum  readily :  second, 
wood. 


FIG.  76.  —  Illustrating  the  use  of  conductors  and  nonconductors  in  cooking 

utensils. 

they  do  not  melt  readily ;  third,  they  do  not  break  when  struck  ; 
fourth,  they  do  not  crack  when  subjected  to  sudden  changes 
of  temperature ;  fifth,  they  conduct  heat  readily.  Of  the 
common  metals  it  will  be  noticed  in  the  table  above  that  copper 
is  the  best  conductor,  and  that  aluminium  comes  next.  Alu- 


io8 


PHYSICS  OF  THE  HOUSEHOLD 


minium  has  an  advantage  in  its  lightness.  The  density  of  copper 
is  8.9  and  of  aluminium  is  2.58,  therefore  copper  is  about  3^ 
times  as  heavy  as  aluminium. 

Handles.  —  Wood  and  porcelain  are  used  for  the  handles  of 
cooking  utensils  (see  i  and  2,  Fig.  76),  because  they  are  poor 
conductors  of  heat.  A  glance  at  the  table  above  shows  us  that 
wood  makes  a  much  better  handle  than  porcelain,  because  it  is 
a  much  poorer  conductor  of  heat. 

A  felt  or  cotton  wool  pad  is  frequently  used  to  cover  the 
handle  of  a  flatiron,  and  to  handle  other  hot  bodies.  Felt 
and  cotton  wooLj.j-.ye  excellent  for  this  purpose  because  they 
have  low  specific  conductivities. 

The  pieces  of  horn  inserted  in  the  handles  of  a  coffeepot 
(see  3,  Fig.  76),  check  the  movement  of  heat  towards  the  handle. 
It  is  the  best  hard  solid  for  this  purpose  because  it  has  the 
smallest  conducting  power. 

The  wire  handle  on  pokers  and  other  appliances  used  about 
the  stove  makes  a  fairly  cool  handle,  because  the  wire  makes 
a  long  path  for  the  heat  to  travel,  and  also  it  has  a  large  sur- 
face from  which  the  heat  radiates. 

The  fireless  cooker. — The  fireless  cooker,  Fig.  77,  is  a  box  in 
which  one  or  more  pails  are  surrounded  by  a  thick  layer  of  non- 
conducting material. 
The  substance  to  be 
cooked  is  placed  in 
a  pail  and  heated 
thoroughly.  The 
pail  is  then  placed 
in  the  box,  and  the 
heat  contained  in 
the  substance  slowly 

completes  the  operation  of  cooking.  The  nonconducting 
material  may  be  felt,  feathers,  mineral  wool,  cotton,  straw, 
shavings,  etc.  The  nonconducting  property  of  these  sub- 
stances is  due  largely  to  the  air  they  contain.  Each  material 


(jVon  Conduct  n$  Hater  &  ™                I 

P&'ll 

Pail 

FbiL 

t 

^ 

1          , 

^7b/^  Conducting  J^TdTf^^^l  —  """^ 

u                   u 

FIG.  77.  —  The  fireless  cooker. 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES 


109 


gives  the  best  results  with  a  certain  closeness  of  packing.  It 
should  be  loose  enough  to  contain  air,  and  still  close  enough  to 
prevent  convection  currents  in  the  air. 

The  refrigerator.  —  The  fireless  cooker  is  designed  to  keep 
heat  in  and  the  refrigerator  is  designed  to  keep  heat  out.  They 
are,  however,  similar  in  principle  and  somewhat  similar  in  con- 
struction. The  refrigerator  is  a  box  with  double  walls,  the  space 
between  the  walls  being  filled  with  a  nonconducting  material 
such  as  is  used  in  the  fireless  cooker.  We  shall  study  the  refrig- 
erator further  in  a  later  section. 

The  thermos  bottle.  —  The  thermos  bo.  _Ae  is  designed  to 
keep  heat  in  or  out,  as  desired.  It  is  different  in  principle 


Glass  Flash 

double  watts 


,7h. 


Outer  Case 


between 


;  -Silver  Co<3/7/25>v5 


Inner 

FIG.  78.  —  The  thermos  bottle. 


from  the  fireless  cooker  and  the  refrigerator.  It  consists  of 
two  bottles  blown  one  inside  the  other  and  sealed  together  at  the 
neck,  Fig.  78.  The  outside  of  the  inner  bottle,  and  the  inside 
of  the  outer  bottle  are  silvered,  then  the  air  is  pumped  out  of 


110  PHYSICS  OF  THE  HOUSEHOLD 

the  space  between  the  bottles,  and  this  space  is  sealed  air- 
tight. 

It  will  be  remembered  that  there  are  three  ways  in  which  heat 
moves  from  one  place  to  another,  namely,  by  conduction,  by 
convection,  and  by  radiation.  It  is  difficult  for  heat  to  enter  or 
leave  the  thermos  bottle  in  any  of  these  ways. 

Let  us  first  consider  that  the  bottle  contains  a  cold  substance, 
and  determine  how  heat  is  kept  out  of  the  bottle.  Heat  does 
not  pass  through  the  sides  of  the  bottle  by  radiation  because, 
when  the  heat  waves  reach  the  bottle,  they  are  reflected  by  the 
bright  silvered  surfaces  in  the  same  way  that  light  is  reflected 
by  the  silvered  surface  of  a  mirror.  A  small  amount  of  heat, 
however,  can  pass  into  the  bottle  by  radiation  down  the  neck. 

Heat  does  not  enter  the  bottle  by  convection  because  there 
is  no  gas  or  liquid  in  the  space  between  the  bottles  in  which  con- 
vection currents  can, form.  It  does  not  enter  the  neck  of  the 
bottle  by  convection  because  the  substance  is  cold  and  the  air 
in  contact  with  it  is  colder  and  heavier  than  the  warmer  outer 
air,  therefore  no  convection  currents  can  occur. 

Heat  does  not  pass  through  the  sides  of  the  bottle  by  conduc- 
tion, because  there  is  a  vacuum  between  the  bottles,  and  a 
vacuum  does  not  carry  heat  by  conduction.  Heat  does,  however, 
enter  the  bottle  by  conduction  down  the  glass  neck  of  the  bottle. 
This  is  a  slow  process,  because  glass  is  a  poor  conductor  of  heat. 

The  neck  of  the  bottle  then  is  the  door  by  which  heat  enters 
the  bottle.  It  enters  here  slowly,  partly  by  conduction  and 
partly  by  radiation.  It  is  probable  also  that  a  little  heat  is 
absorbed  by  the  silver  surface  of  the  outer  bottle  and  is  con- 
ducted by  the  silver  to  the  coating  of  the  inner  bottle  and  thus 
through  the  glass  to  the  substance. 

If  we  consider  the  bottle  to  be  filled  with  a  hot  substance,  and 
think  of  the  ways  in  which  heat  is  retained,  we  find  that  it  is 
retained  in  each  of  the  ways  that  it  is  kept  out  when  the  sub- 
stance is  cold.  Heat  escapes,  however,  in  one  more  way  than  it 
enters  the  bottle.  It  escapes  by  convection  in  the  air  in  the  neck 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES  III 

of  the  bottle.  It  is  a  matter  of  observation  that  the  thermos 
bottle  keeps  a  substance  cool  longer  than  it  keeps  a  substance 
hot,  and  the  reason  is  that  given  above.  Heat  can  escape  from 
the  bottle  in  one  more  way  than  it  can  enter. 

Walls  of  houses. — The  walls  of  houses  serve  to  keep  heat  in, 
in  winter,  and  out,  in  summer.  They  are  made  of  layers  of 
nonconducting  material  with  a  layer  of  air  between  them. 
When  the  air  in  a  room  is  heated,  part  of  the  heat  is  transferred 
to  the  walls  and  furniture.  Any  one  in  the  room  is  warmed 
partly  by  heat  from  the  air,  and  partly  by  heat  radiated  from 
the  walls  and  funiture.  The  walls  help  to  keep  heat  in  as  fol- 
lows. The  inside  surface  of  the  inner  layer  absorbs  heat  from 
the  air,  and  radiates  part  of  it  back  into  the  room.  Part, 
however,  is  conducted  through  the  layer  and  transferred  to 
the  second  layer  by  radiation  and  convection.  The  inside 
surface  of  the  outer  layer  absorbs  this  heat  and  radiates  part 
of  it  back  to  the  inner  layer.  Part  of  it  is,  however,  conducted 
through  the  layer  and  escapes  by  radiation  and  convection 
into  the  outer  air. 

Heat  is  kept  out  in  summer  by  the  reverse  operation. 

Clothes.  —  Clothes  are  made  of  nonconducting  material 
with  layers  of  air  between  them.  They  help  to  keep  us  warm 
in  winter  and  cool  in  summer.  Our  bodies  are  warmed  from 
within  by  the  heat  produced  by  the  oxidation  of  the  food  we 
eat.  Every  one  radiates  heat  at  all  times.  If  heat  waves 
produced  the  sensation  of  sight  as  light  waves  do,  every  person 
would  be  luminous.  The  heat  radiated  by  the  body  is  partly 
absorbed  by  the  clothing  nearest  the  body.  Some  of  this  is 
radiated  back  to  the  body,  and  some  passes  on  to  the  next  gar- 
ment. This  process  is  repeated  at  each  garment. 

Ventilation.  —  The  ventilation  of  dwellings  is  brought  about 
by  convection  currents  in  air.  These  convection  currents 
can  be  detected  by  means  of  experiments  similar  to  that 
illustrated  in  Fig.  79.  If  a  door  leading  from  a  warm  to 
a  cold  room  is  opened,  cold  air  enters  the  warm  room  at 


112 


PHYSICS  OF  THE  HOUSEHOLD 


the  bottom  and  forces  the  warm  air  out  at  the  top.  A 
lighted  candle  held  in  different  positions  indicates  the 
direction  of  these  currents.  The  flame  of  the  candle  a  is 
blown  towards  the  warm  room,  the  direction  in  which  the  cold 
air  moves.  The  candle  c  is  blown  towards  the  cold  room,  the 
direction  in  which  the  warm  air  moves.  The  flame  of  the 
candle  b  is  stationary,  or  moves  first  in  one  direction  then  in 

the  other,  because  the  air  at  this 
point  is  between  the  warm  and  cold 
currents. 

Similarly,  if  the  window  of  a  warm 
room  is  opened  at  the  top  and  bot- 
tom on  a  still,  cold  day,  a  lighted 
candle  shows  that  air  enters  at  the 
bottom  and  leaves  at  the  top.  Cold 
air  enters  at  the  bottom,  and  forces 
warm  air  out  at  the  top. 

A  strong  wind,  blowing  against 
one  side  of  the  house,  forces  fresh 
air  into  the  house  through  the 
crevices  about  the  windows  and 
doors  on  this  side  of  the  house. 


FIG.  79.  —  Convection  currents 
between  hot  and  cold  rooms. 


The  foul  air  is  then  forced  out 

through  the  crevices  about  the  doors  and  windows  on  the  other 
sides.  Thus  the  house  is  ventilated. 

Other  means  of  ventilation.  —  A  lighted  stove  provides  a  cer- 
tain amount  of  ventilation,  because  air  from  the  room  enters 
the  stove  to  make  the  draft.  This  decreases  the  pressure  of 
the  air  in  the  room  slightly.  Fresh  air  from  outside  is  then 
forced  into  the  room  through  the  crevices  about  the  doors  and 
windows  by  atmospheric  pressure.  Thus  a  continuous  supply 
of  fresh  air  is  forced  into  the  room,  and  the  room  is  ventilated. 

A  lighted  grate  or  grate  stove  is  a  still  better  means  of  venti- 
lation. A  certain  amount  of  the  air  of  the  room  enters  the  grate 
below  the  fire  to  make  the  t  draft,  but  a  still  larger  amount 


MOVEMENT  OF  HEAT.     HEAT  APPLIANCES 


enters  above  the  fire  and  passes  up  the  chimney.     Fresh  air 
is   then  forced  in  from  outside  to  take  the  place  of  this  air, 


Ag™* 


FIG.  80.  —  One  method  of  ventilating  a  house. 

and  thus  the  room  is  ventilated.     The  grate  and  grate  stove 
provide  excellent  ventilation. 

One  method  of  bringing  fresh  air  into  the  house  is  illustrated 
in  Fig.  80.  Cold  air  from  outside  is  heated  by  steam  or  hot- 
water  pipes  and  is  forced  up  into  the  rooms  above  by  more  cold 
air  from  outside. 

Another  method  is  illustrated  in 
Fig.  8 1 ;  cold  air  from  outside  is  ad- 
mitted beneath  a  radiator  which  is 
inclosed  except  at  the  top.  The  air 
enters  as  described  above. 

Ventilating  a  soil  pipe.  —  The  ordi- 
nary soil  pipe  is  a  4-in.  cast-iron  pipe. 
It  carries  the  wastes  from  the  sink, 
bath,  water  closet,  etc.,  to  the  cess- 
pool, septic  tank,  or  sewer.  It  is  the 
usual. practice  to  have  an  opening  at 
1  i 


FIG.  81.  —  Ventilating  by 
means  of  a  radiator. 


114 


PHYSICS  OF  THE  HOUSEHOLD 


the  bottom  and  top  of  this  pipe 
to    provide    ventilation.      The 
arrangement  of  the  soil  pipe  is 
shown  in  Fig.  82.     It  will  be 
noticed  that  there  is  a  fresh-air     ^> 

ROOF  VENT  ^^^ 

^ 

inlet  near  the  bottom,  and  that 
the  pipe  extends   through  the 
roof   and  is  open  at  the  top. 
The  ventilation  is  produced  by 
convection  as  follows  :  The  air 
in  the  pipe  is  warmer  than  the 
air  outside  the  house,  therefore 
the  air  in  the  pipe  is  lighter, 
volume   for  volume,   than   the 
air  outside.     Thus  the  cold  air 
sinks  down  into  the 

e         i          •       •     i     ,               j                 FRESH  AIR  INLET 

iresn-air  inlet,  and        s 

p                                                                     <^S           GROUND  LINE 

.  WATER  CLOSET 

<3^^ 

SOIL  PIPE 

*s 

BATH    ROOM 

WASH  BOWL 
±TRAP 

1  f 

•i 
? 

|    BATH  TUBy^ 

jsgr 

_T3I" 

^  BATH  TRAP 

forces  the  warmer     ~f~ 
air  out  at  the  roof     ^                n  ; 

CELLAR 
«  TRAP  IN  SOIL  PIPE 

Vent.        The       COld               VENTILATING  PIPE 

air  is  warmed  and 
is    forced    out    by  ^^Zrft  CESS  P^2 

1  j          •                   j                      OR  SEPTIC  TANK 

more,  odd  air,  and         RG.  82._The 
so  on. 

—  IW- 

^ 

soil  pipe  and  its  ventilation. 

EXERCISES 

1.  Describe  how  heat  moves  by  conduction. 

2.  Describe  an  experiment  to  show  that  liquids  are  poor  conductors 
of  heat. 

3.  Describe  how  heat  moves  by  convection. 

4.  Name  some  household  appliances  in  which  heat  moves  by  con- 
vection. 

5.  Describe  how  heat  moves  from  one  place  to  another  by  radiation. 

6.  Name  three  examples  of  the  transfer  of  heat  by  radiation. 

7.  Define  specific  conductivity. 

8.  Why  are  cooking  utensils  made  of  metal? 

9.  Describe  the  fireless  cooker. 


MOVEMENT  OF  HEAT.     HEAT   APPLIANCES  115 

10.  Describe  how  the  thermos  bottle  is  made. 

11.  Describe  how  the  thermos  bottle  keeps  heat  away  from  a  cold 
substance. 

12.  Describe  how  the  thermos  bottle  retains  the  heat  of  a  hot  sub- 
stance. 

13.  Describe  how  the  walls  of  a  house  help  to  keep  the  heat  in. 

14.  Describe  how  clothes  help  to  retain  the  heat  of  the  body. 

15.  Describe  three  ways  in  which  a  room  can  be  ventilated. 

16.  Draw  a  soil  pipe  and  describe  how  it  is  ventilated. 

17.  Test  a  fireless  cooker  by  measuring  the  amount  of  heat  which  passes 
out  through  the  walls  in  a  given  time,  as  follows :    Weigh  the  largest  pail. 
Pour  into  it  10  pounds  of  water,  put  on  the  cover  undamped,  and  heat  until 
the  water  boils.  *  Place  the  pail  in  the  fireless  cooker,  clamp  the  cover,  close 
the  cooker,  and  allow  it  to  stand  for  one  half  hour.     This  allows  the  cooker 
to  warm  up.     Open  the  cooker  and  take  the  temperature  of  the  water. 
Close  the  cooker  and  allow  it  to  stand  for  5,  10,  or  20  hours.     Find  the  tem- 
perature of  the  water.     Calculate  the  number  of  B.  T.  U.  lost  in  the  time 
chosen. 


CHAPTER  X 
MEASUREMENT  OF  HEAT 

Heat  units  and  how  to  use  them.  —  We  find  the  temperature 
of  a  body  by  means  of  a  thermometer,  but  the  temperature 
alone  does  not  indicate  the  quantity  of  heat  in  a  body.  Two 
bodies  may  be  at  the  same  temperature  and  still  contain  very 
different  quantities  of  heat.  We  all  know,  for  example,  that 
a  much  smaller  quantity  of  heat  is  required  to  warm  a  cup  of 
water  to  the  boiling  point  than  to  warm  a  teakettle  of  water  to 
the  same  temperature. 

If  you  were  asked  to  measure  out  a  pound  of  heat  or  a  quart 
of  heat,  you  would  find  it  impossible  to  do  so.  You  could, 
however,  measure  out  a  pound  or  quart  of  some  hot  substance. 
This  is  the  method  we  use  to  measure  quantity  of  heat;  we 
measure  the  weight  and  temperature  of  some  hot  substance. 
Liquids  are  convenient  for  this  purpose,  and  water,  being  the 
most  common  liquid,  is  the  one  generally  used. 

In  engineering  work  in  Great  Britain  and  North  America,  the 
unit  quantity  of  heat  is  the  amount  of  heat  required  to  raise  the 
temperature  of  i  Ib.  of  water  i°  F.  It  is  called  the  British 
Thermal  Unit  (B.  T.  U.). 

In  scientific  work  in  all  countries,  the  unit  quantity  of  heat  is 
the  amount  of  heat  required  to  raise  the  temperature  of  i  g.  of 
water  i°  C.  It  is  called  the  calorie. 

Also,  if  i  Ib.  of  water  cools  i°  F.,  it  gives  up  i  B.  T.  U.  of  heat, 
and  if  i  g.  of  water  cools  i°  C.  it  gives  up  i  calorie  of  heat,  etc. 

Experiment  3.     Heat  units. 

Object.  To  illustrate  the  meaning  of  the  terms  "  British  Thermal 
Unit  "  and  "  calorie." 

116 


MEASUREMENT  OF  HEAT 


117 


FIG.  83.  —  Measuring  heat. 


Method.  The  British  Thermal  Unit.  In  a  vessel  such  as  a  saucepan 
or  3-qt.  pail,  weigh  out  2  Ib.  of  cold  water.  Take  the  temperature  of 
the  water  in  degrees  Fahrenheit. 

Place  the  vessel  on  the  fire  for  a  certain  time,  say  3  min.,  remove  ifr 
from  the  fire  and   find  the  temperature  of 
the  water  in  degrees  Fahrenheit. 

Calculate  the  number  of  B.  T.  U.  which 
the  2  Ib.  of  water  received  in  3  min. 

Example.  If  the  2  Ib.  of  water  is  warmed 
25°  F.  the  water  receives  2  X  25  =  50 
B.  T.  U.  of  heat. 

Place  the  hot  water  in  a  cool  place  for  a 
certain  time,  say  3  min.  Find  its  tempera- 
ture. 

Calculate  the  number  of  B.  T.  U.  the  water 
lost  in  3  min. 

The  Calorie.  Weigh  out  500  g.  of  cold 
water.  Find  its  temperature  in  degrees 
centigrade. 

Place  it  on  the  fire  for  a  certain  time  and  again  find  its  temperature. 

Calculate  the  number  of  calories  the  water  received. 

Example.  If  the  500  g.  of  water  is  warmed  30°  C,  it  receives  500  X 
30  =  1500  calories  of  heat. 

Place  the  water  in  a.cool  place  for  a  certain  time.  Find  its  tempera- 
ture. 

Calculate  the  number  of  calories  the  water  lost. 

Record.    In ....  minutes  the  water  gained B.T.U. 

In minutes  the  water  lost B.T.U. 

In ...  .minutes  the  water  gained - calories. 

In . .  .  .  minutes  the  water  lost calories. 

The  B.  T.  U.  and  calorie.  —  In  the  experiment  above  we  have 
gained  some  experience  in  measuring  heat  by  means  of  the  heat 
units, British  Thermal  Unit  (B.T.U.)  and  calorie. 

Examples.  —  If  4  Ib.  of  water  is  warmed  from  55°  F.  to  85°  F. 
the  water  receives  4  X  30  =  120  B.  T.  U.  of  heat. 

If  4  Ib.  of  water  cools  from  85°  F.  to  70°  F.  it  loses  4X15  = 
60  B.T.U.  of  heat. 

If  300  g.  of  water  is  warmed  from  25°  C.  to  45°  C.,  it  receives 
300  X  20  =  6000  calories  of  heat. 


n8 


PHYSICS  OF  THE  HOUSEHOLD 


If  300  g.  of  water  cools  from  45°  C.  to  30°  C.,  it  loses  300  X  i 
4500  calories  of  heat. 

We  shall  gain  further  experience  in  using  heat  units  in  later 
experiments. 

It  may  be  noted  that  another  heat  unit,  the  greater  calorie 
or  kilogram  calorie,  the  amount  of  heat  required  to  raise  the 
temperature  of  one  kilogram  of  water  one  degree  centigrade,  is 
also  in  common  use.  Often  the  greater  or  kilogram  calories  are 
spoken  of  simply  as  calories.  Students  of  household  science 
are  apt  to  meet  this  use  of  the  term  in  books  on  food  and  utri- 
tion.  Since  the  greater  calo  ;  is  a  thousand  times  the  lesser 
calorie,  there  will  usually  be  )  difficulty  in  judging  from  the 
context  which  is  intended. 

Experiment  4.     Comparing       js  and  cooking  utensils. 


Part  I 

Object.  To  compare  the  heating  effect  of  one  gas  burner  with  that 
of  another. 

Method.  In  a  vessel,  such  as  a  saucepan,  weigh  out  2  or  3  Ib.  of  cold 
water.  Find  the  temperature  of  the  water  in  degrees  Fahrenheit. 

Place  the  vessel  over  one  gas  burner  for  a  certain  time,  say  5  min. 


igJBunsen  C  ring  burners        Comparing  Cootonb  -utensils 


FIG.  84.  —  Comparing  the  amounts  of 
conducted  by  tw 


y  two  fires,  and  the  amounts 
ensils. 


MEASUREMENT  OF  HEAT  1 19 

T'ind  the  temperature  of  the  water,  and  calculate  the  number  of 

.  U.  of  heat  which  the  fire  gave  to  the  water. 

apeat  the  experiment,  using  the  same  vessel  on  the  second  burner. 
Which  burner  gave  the  greater  amount  of  heat  to  the  water? 

Part  II 

Object.  To  compare  the  conductivity  of  one  vessel  with  that  of 
another. 

Method.  Let  us  suppose  we  wish  to  compare  the  conductivity  of  a 
copper  saucepan  with  that  of  a  granite  ware  saucepan  of  the  same  shape 
and  si  -e. 

Weigh  out  a  certain  weight  of  cold  water  in  the  copper  vessel,  say 
2  Ib.  Find  the  temperature  of  the%kl  water  in  Fahrenheit  degrees. 
Place  it  on  the  fire  for  a  certain  time*f%ay  4  min.  Find  the  temperature 
of  the  water  again  and  calculate  the  number  of  B.  T.  U.  which  passed 
through  the  bottom  of  the  copper  vessel  into  the  water. 

Repeat  the  experiment,  using  theN^franite  ware  vessel. 

WTiich  vessel  is  the  better  conductor  of  heat? 

Record.     Part  I. 

Burner  (i)  gave B.  T.  U.  in minutes. 

Burner  (2)  gave ........  B.  T.  U.  in minutes. 

Burner gave  the  greater  amount  of  heat. 

Part  II. 

Vessel  (i)  absorbed B.  T.  U.  in minutes. 

Vessel  (2)  absorbed B.  T.  U.  in minutes. 

Vessel is  the  better  conductor  of  heat. 

Comparing  fires.  —  In  the  experiment  above  we  used  the  heat 
units  to  compare  the  heating  effect  of  one  fire  with  that  of 
another. 

If  one  fire  warms  5  Ib.  of  water  from  60°  F.  to  95°  F.  in  a  cer- 
tain time,  and  the  other  fire  warms  5  Ib.  of  water  from  60°  F.  to 
90°  F.  in  the  same  length  of  time,  we  know  that  the  first  fire 
gives  more  heat  than  the  second.  The  first  fire  gives  the  water 
5  X  35  =  175  B.T.  U.  of  heat,  and  the  second  5  X  30  =  150 
B.T.  U.  of  heat  in  the  same  length  of  time. 

In  the  experiment  we  ce^nared  the  heating  effect  of  one  gas 
burner  with  that  of  anc '  nvever,  we  wish  to  determine 

which  burner  is  the  more  .cal,  we  must  measure  the  quan- 


120  PHYSICS   OF  THE  HOUSEHOLD 

tity  of  gas  used  by  each  burner  in  a  certain  time,  as  well  as  the 
amount  of  heat  produced  in  this  time. 

Comparing  cooking  utensils.  —  In  the  experiment  above  we 
learned  also  how  to  use  the  heat  units  to  compare  the  con- 
ductivity of  one  utensil  with  that  of  another. 

If  in  one  utensil  5  Ib.  of  water  is  warmed  from  60°  F.  to  100°  F. 
in  a  certain  time,  and  in  the  other  5  Ib.  of  water  is  warmed  from 
60°  F.  to  80°  F.,  on  the  same  fire  and  in  the  same  length  of  time, 
we  know  that  the  first  utensil  has  the  greater  conductivity. 
The  first  utensil  conducts  5X40=  200  B.T.  U.  of  heat, 
and  the  second  5  X  20  =  looB.T.  U.  of  heat,  from  the  same 
fire  in  an  equal  time. 

This  method  can  be  used  to  measure  the  conductivity  of 
cooking  utensils  in  general. 

Comparing  fireless  cookers.  —  We  can  also  use  the  heat 
units  in  comparing  the  heat-holding  power  of  one  fireless  cooker 
with  that  of  another,  as  follows  :  Place  equal  weights  of  boiling 
water  (212°  F.)  in  each  fireless  cooker.  Find  the  temperature 
of  the  water  in  each  after  a  certain  time,  say  i  hr.  Calculate 
the  quantity  of  heat  lost  through  the  sides  of  each  fireless  cooker 
in  the  given  time.  The  one  which  loses  the  smaller  quantity 
of  heat  has  the  greater  heat-holding  power,  and,  other  things 
being  equal,  is  the  better  fireless  cooker. 

Experiment  5.     Foot  warmers. 

Object.  To  compare  the  amount  of  heat  given  up  by  two  different 
foot  warmers  which  have  the  same  weight  and  are  at  the  same  tempera- 
ture. 

Let  us  suppose  that  we  wish  to  find  out  which  makes  the  better  foot- 
warmer,  a  flatiron  or  the  water  in  a  hot-water  bag,  the  iron  and  water 
to  have  the  same  weight  and  to  be  at  the  same  temperature. 

Method.  Balance  a  pail  on  one  pan  of  the  scales.  Then  place  the 
flatiron  on  the  other  pan  and  pour  enough  water  into  the  pail  to  balance 
the  iron. 

We  now  have  equal  weights  of  iron  and  water. 

Place  the  iron  in  the  water,  cover  the  pail,  and  heat  the  iron  and  water 
to  the  boiling  point  of  water. 


MEASUREMENT  OF  HEAT  121 

We  now  have  equal  weights  of  iron  and  water  both  at  212°  F. 

In  each  of  two  other  pails  weigh  out  a  certain  weight  of  cold  water, 
say  3  Ib.  Find  the  temperature  Fahrenheit  of  the  water  in  each  pail. 

Place  the  hot  iron  in  one  pail,  and  the  hot  water  in  the  other. 

Find  the  temperature  of  the  water  in  each  pail  after  i  or  2  min. 

Calculate  the  number  of  B.  T.  U.  the  hot  iron  gave  to  the  cold  water, 
and  the  number  of  B.  T.  U.  the  hot  water  gave  to  the  cold  water. 

Eoual  weights 
of  Iron  c  water 
heated  to 


Eoual  weights   of 

Cold  wster 


FIG.  85.  —  Comparing  iron  and  water  as  foot  warmers. 

Example.  If  the  hot  iron  warmed  the  3  Ib.  of  cold  water  15°  F.,  the 
iron  gave  up  3  X  15  =  45°  B.  T.  U.,  etc. 

Which  foot  warmer  gave  up  the  greater  amount  of  heat,  that'  is,  which 
is  the  better  foot  warmer  ? 

You  will  find  that  one  substance  gives  up  much  more  heat  than  the 
other.  This  substance  is  said  to  have  the  greater  heat  capacity. 

Record.  The  hot  iron  warmed  the Ib.  of  cold  water °  F., 

therefore  the  hot  iron  gave  the  cold  water B.  T.  U. 

The  hot  water  warmed  the  . .  .  .Ib.  of  cold  water. .  . .°  F.,  therefore 
the  hot  water  gave  the  cold  water .  .  . .  B.  T.  U. 

The is  the  better  foot  warmer. 

Comparing  foot  warmers.  —  In  the  experiment  above  we  used 
the  heat  units  in  comparing  foot  warmers.  A  good  foot  warmer 
is  one  that  stores  up  a  large  quantity  of  heat  which  it  gives  up 
gradually.  We  compared  two  foot  warmers,  namely:  a  flat- 
iron  and  the  water  in  a  hot-water  bag.  We  found  that  the  hot 
water  gave  up  much  more  heat  than  the  hot  iron.  We  con- 
cluded then  that  the  water  in  a  hot-water  bag  is  a  better  foot 
warmer  than  a  flatiron  of  equal  weight  heated  to  the  same  tern- 


122 


PHYSICS   OF  THE  HOUSEHOLD 


perature.  Water  is  a  better  material  for  a  foot  warmer  than 
iron,  because  water  has  a  greater  capacity  for  heat  than  iron. 
This  will  be  taken  up  further  in  a  later  section. 

Experiment  6.     Cooling  effect  of  ice  and  ice  water. 

Object.  To  compare  the  cooling  effect  of  i  Ib.  of  ice  at  32°  F.,  with 
that  of  i  Ib.  of  ice  water  at  32°  F. 

Method.  Ice  water.  Weigh  out  2  Ib.  of  water,  cover  the  pail,  and 
heat  the  water  to  the  boiling  point,  212°  F.  Pour  into  this  i  Ib.  of  ice 
water  and  find  the  temperature  after  stirring  for  one  minute. 


Zlbs 


lib  of  ice  water  poured 
info  Zlbs.  bailing 


lib  of  ice  poured 
tnto  22bs. 


FIG.  86.  —  Comparing  the  cooling  effect  of  a  pound  of  ice  with  that  of  a 
pound  of  ice  water. 

Calculate  the  number  of  B.T.  U.  which  the  ice  water  took  from  the 
2  Ib.  of  hot  water. 

Example.  If  the  2  Ib.  of  hot  water  at  212°  was  cooled  to  152°,  the 
ice  water  took  2  X  60  =  120  B.T.  U.  of  heat  from  the  hot  water. 

This  represents  the  cooling  effect  of  i  Ib.  of  ice  water. 

Ice.  Weigh  out  2  Ib.  of  water  and  heat  it  to  the  boiling  point,  212°  F. 
Put  into  this  i  Ib.  of  ice  or  snow.  Take  the  temperature  after  all  the 
ice  is  melted. 

Calculate  the  number  of  B.T.  U.  which  the  ice  took  from  the  2  Ib.  of 
hot  water. 

This  represents  the  cooling  effect  of  i  Ib.  of  ice. 

Which  has  the  greater  cooling  effect,  i  Ib.  of  ice  water  or  i  Ib.  of  ice? 

Note.  To  make  ice  water,  put  enough  snow  or  ice  in  water  to  reduce 
its  temperature  to  32°  F.  Then  strain  the  water  through  a  cloth  to 
remove  the  ice  which  is  not  melted. 


MEASUREMENT  OF  HEAT 


I23 


Record. 

i  Ib.  of  ice  water  cooled  2  Ib.  of  water  from  2 1 2°  F.  to °  F.  There- 
fore the  i  Ib.  of  ice  water  absorbed . .  .  .B.  T.  U. 

i  Ib.  of  ice  cooled  2  Ib.  of  water  from  212°  F.  to  .  .  .  .°  F.  Therefore 
the  i  Ib.  of  ice  absorbed B.  T.  U. 

Therefore  i  Ib.  of has  a  greater  cooling  effect  than  i  Ib. 

of 

Ice  and  ice  water.  —  We  can  use  the  heat  units  in  meas- 
uring the  cooling  effect  of  different  materials.  In  the  experi- 
ment abov^e  we  compared  the  cooling  effect  of  i  Ib.  of  ice  water 
at  32°  F.,  with  that  of  i  Ib.  of  ice  at  32°  F.  We  found  that  ice  has 
a  much  greater  cooling  effect  than  an  equal  weight  of  ice  water 
at  the  same  temperature.  Ice  has  a  greater  cooling  effect 
than  ice  water  because  a  large  quantity  of  heat  is  required  to 
change  the  ice  to  ice  water.  To  change  i  Ib.  of  ice  at  32°  F.  to 
i  Ib.  of  water  at  32°  F.  requires  144  B.  T.  U.  of  heat.  This  is 
known  as  the  latent  heat  of  ice.  We  shall  study  it  further  in 
a  later  section. 

Experiment  7.     The  heating  effect  of  steam  and  of  boiling  water. 

Object.  To  compare  the  heating  effect  of  i  Ib.  of  steam  at  212°  F. 
with  that  of  an  equal  weight  of  water  at  212°  F. 

Method.  Boiling  water.  Weigh  out  4  Ib.  of  cold  water  and  find  its 
temperature.  Leave  the  vessel  and  water  on  the  scales  and  pour  into 
it  \  Ib.  of  water  at  212°  F.  and  find  the  temperature  again. 


'/libs 


FIG.  87.  —  Comparing  the  heating  effects  of  steam  and  boiling  water. 


124  PHYSICS  OF  THE  HOUSEHOLD 

Calculate  the  number  of  B.  T.  U.  received  by  the  4  Ib.  of  cold  water. 
This  is  the  heating  effect  of  the  \  Ib.  of  water  at  212°  F. 

Steam.     Weigh  out  4  Ib.  of  cold  water  and  take  its  temperature. 

Place  the  vessel  and  water  on  a  balance  and  note  its  weight.  Leave 
it  on  the  balance  and  pass  live  steam  into  the  water  until  \  Ib.  of  steam 
has  condensed  in  the  cold  water.  Find  the  temperature  again. 

Calculate  the  number  of  B.  T.  U.  received  by  the  4  Ib.  of  cold  water. 

This  is  the  heating  effect  of  the  J  Ib.  of  steam. 

Which  has  the  greater  heating  effect,  water  at  212°  F.  or  the  same 
weight  of  steam  at  212°  F.? 

Record.  The  \  Ib.  of  boiling  water  warmed  4  Ib.  of-  cold  water 

from °  F.  to °  F.  Therefore  the  I  Ib.  of  boiling  water  gave 

up..  ..B.T.  U. 

The  £  Ib.  of  steam  warmed  4  Ib.  of  cold  water  from °  F.  to 

°  F.  Therefore  the  i  Ib.  of  steam  gave  up B.  T.  U. 

A  certain  weight  of has  a  greater  heating  effect  than  the  same 

weight  of 

Steam  and  boiling  water.  —  We  can  use  the  heat  units  in 
measuring  the  heating  effect  of  any  material.  In  the  experi- 
ment above  we  compared  the  heating  effect  of  i  Ib.  of  boiling 
water  with  that  of  i  Ib.  of  steam  at  the  same  temperature.  We 
found  that  steam  has  a  much  greater  heating  effect  than  an 
equal  weight  of  water  at  the  same  temperature.  Steam  has  a 
greater  heating  effect  than  water  at  the  same  temperature  be- 
cause steam  gives  up  a  large  quantity  of  heat  when  it  changes 
from  steam  at  212°  F.  to  water  at  212°  F.  When  i  Ib.  of 
steam  at  212°  F.  changes  to  i  Ib.  of  water  at  212°  F.  it  gives 
up  966  B.  T.  U.  of  heat.  This  is  known  as  the  latent  heat  of 
steam.  We  shall  study  it  further  in  the  next  section. 

EXERCISES 

1.  Define  British  Thermal  Unit  and  calorie. 

2.  What  quantity  of  heat  is  required  to  warm  12  Ib.  of  water  from 
55°  F.  to  205°  F.? 

3.  What  quantity  of  heat  is  required  to  warm  600  g.  of  water  from 
20°  C.  to  100°  C.? 

4.  On  one  fire,  4  Ib.  of  water  is  warmed  from  55°  F.  to  85°  F.  in  5  min. 
On  another  4  Ib.  of  water  is  warmed  from  55°  F.  to  80°  F.  in  the  same 


MEASUREMENT  OF  HEAT  125 

vessel  and  in  an  equal  time.     How  much  heat  does  each  fire  give  to  the 
water  ? 

5.  In  one  saucepan  5  Ib.  of  water  is  warmed  from  60°  F.  to  110°  F.  in 
a  certain  time.     In  another  4  Ib.  of  water  is  warmed  from  60°  F.  to  1 20°  F. 
on  the  same  fire  in  an  equal  time.     What  quantity  of  heat  passes  through 
the  bottom  of  each  vessel  in  the  given  time  ?     Which  vessel  is  the  better 
conductor  of  heat? 

6.  In  one  fireless  cooker  8  Ib.  of  water  at  212°  F.  cools  to  180°  F.  in 
a  certain  time.     In  another  8  Ib.  of  water  at  212°  F.  cools  to  175°  F.  in 
an  equal  time.     What  quantity  of  heat  is  lost  through  the  sides  of  each 
fireless  cooker  in  the  given  time?     Which  is  the  better  heat  retainer? 

7.  Why  is  water  a  better  material  for  a  foot  warmer  than  iron? 

8.  Why  has  ice  a  greater  cooling  effect  than  an  equal  weight  of  ice 
water? 

9.  Why  has  steam  a  greater  heating  effect  than  an  equal  weight  of 
water  at  the  same  temperature? 


CHAPTER  XI 
HEAT   CAPACITY,   SPECIFIC   HEAT,   LATENT  HEAT 


f 

o 


Heat  capacity.  —  The  heat  capacity  of  any  substance  is  the 
amount  of  heat  required  to  warm  unit  weight  of  the  substance  i 
or  the  heat  given  up  when  unit  weight  of  the  substance  cools  i 

If  we  use  British  Thermal  Units  to  measure  heat,  the  heat 
capacity  of  any  substance  is  the  number  of  B<  T.  U.  required 
to  heat  i  Ib.  of  the  substance  i°  F.,  or  what  is  the  same  thing, 
the  number  of  B.T.  U.  given  up  when  i  Ib.  of  the  substance 
cools  i°  F. 

If  we  use  calories  to  measure  the  heat,  the  heat  capacity  of 
any  substance  is  the  number  of  calories  required  to  warm  i  g. 
of  the  substance  i°  C.,  or  the  number  of  calories  given  up  when 
i  g.  of  the  substance  cools  i°  C. 

The  number  expressing  the  heat  capacity  is  the  same,  no 
matter  which  heat  unit  we  use. 

Specific  heat.  —  The  specific  heat  of  a  substance  is  the  ratio 
between  its  heat  capacity  and  that  of  water.  Since,  however, 
the  heat  capacity  of  water  is  i,  the  numbers  expressing  the 
specific  heat  are  the  same  as  those  expressing  heat  capacity. 

Experiment  8.     Heat  capacity. 

Object.     To  find  the  heat  capacity  of  iron. 

Method.  Weigh  a  piece  of  iron  in  pounds,  say  a  flatiron.  Place  it  in 
boiling  water  for  about  five  minutes,  until  its  temperature  becomes  that 
of  boiling  water,  212°  F. 

In  a  separate  vessel  weigh  out  a  certain  quantity  of  cold  water,  say 
5  Ib.  Take  its  temperature  in  degrees  Fahrenheit. 

Place  the  hot  iron  in  the  cold  water  and  find  the  temperature  after 
stirring  for  i  or  2  min. 

126 


HEAT  CAPACITY,   SPECIFIC  HEAT,   LATENT  HEAT      127 


Calculate  the  number  of  B.T.  U.  given  up  by  i  Ib.  of  iron  in  cooling 
i°  Fv  that  is,  calculate  its  heat  capacity. 

Example.  •  4  Ib.  of  iron  at  212°  F.  placed  in  5  Ib.  of  water  at  55°  F. 
warms  the  water  10.67°  F.  What  is  the  heat  capacity  of  iron? 

The  water  absorbed  the  number  of  B.T.  U.  of  heat  which  the  iron 
gave  up.  The  5  Ib.  of  water  was  warmed  from  55°  to  67°  or  through 


4 /As.  of  Iron 
bejn&  w 
to   Zfc  °  F. 


FIG.  88.  —  Measuring  the  heat  capacity  of  iron. 

12°,  therefore  the  water  received  5  X  12  =  60  B.T.U.  This  heat 
came  from  the  iron. 

The  4  Ib.  of  iron  cooled  from  212°  F.  to  67°  F.,  or  through  145°  F., 
therefore  we  can  say : 

4  Ib.  of  iron  in  cooling  145°  gave  up  60  B.  T.  U. 

i  Ib.  of  iron  in  cooling  145°  gave  up  ^  =  15  B.T.U. 

i  Ib.  of  iron  in  cooling  i°  gave  up  jf&  =  ^  B.  T.  U. 

The  i  Ib.  of  iron  in  cooling  i°  gave  up  ^  B.  T.  U.,  therefore  the  heat 
capacity  of  the  iron  is  .1  B.  T.  U. 

Heat  capacity  of  a  substance.  —  The  method  of  finding  the 
heat  capacity  of  a  substance  is  illustrated  in  the  experiment 
above.  We  found  the  heat  capacity  of  iron  to  be  ^  B.  T  .U. 
per  pound.  If  we  make  the  same  experiment  but  measure  the 
weight  of  the  iron  in  grams  and  the  quantity  of  heat  in  calories, 
we  find  the  heat  capacity  of  iron  to  be  TV  calorie  per  gram.  That 
is,  the  number  expressing  the  heat  capacity  is  the  same  in 
each  system  of  measurement. 


128 


PHYSICS  OF  THE  HOUSEHOLD 


In  this  experiment  we  found  the  heat  capacity  of  iron.  The 
heat  capacity  of  other  substances  can  be  found  in  the  same  way. 

Table  of  Heat  Capacities 

Air 25 

Alcohol 6 

Brass 09 

Brick 2 

Copper 09 

Earth 2 

Glass 2 

Ice 5 

Use  of  heat  capacity.  —  When  we  know  the  heat  capacity  and 
weight  of  a  body,  we  can  calculate  the  quantity  of  heat  it  will 
take  up  or  lose  for  a  given  change  of  temperature. 

Example.  A  5-lb.  brick  used  as  a  foot  warmer  cools  from 
200°  F.  to  60°  F.  What  quantity  of  heat  does  it  give  up? 

Ans.     5  X  140  X  .2  =  140  B.T.  U. 


Iron 

jj 

Lead      

.031 

Mercury    . 

•033 

Silver     

-057 

Soapstone  . 

.2 

Tin        

o"\6 

Water 

i 

Zinc. 

•095 

LATENT  HEAT 

Experiment  9.    Latent  heat  of  ice. 

Object.  To  find  the  number  of  B.  T.  U.  required  to  change  i  Ib.  of 
ice  at  32°  F.  to  i  Ib.  of  water  at  32°  F.,  that  is,  to  find  the  latent  heat  of 
ice. 

Method.    Weigh  out  5  Ib.  of  water  in  a  pail  and  warm  it  to  about  80°  F. 

Take  its  temperature. 
Pour  into  it  i  Ib.  of 
dry  powdered  ice  or 
snow. 

Stir  and  take  the 
resulting  temperature 
when  all  the  ice  is 
melted. 

Calculate  the  num- 
ber of   B.  T.  U.   re- 
quired   to    melt    the 
pound  of  ice.     This  is 
FIG.  89.  — Measuring  the  latent  heat  of  ice.  the  latent  heat  of  ice. 


Ittxlce 

<3t 


HEAT  CAPACITY,   SPECIFIC   HEAT,   LATENT  HEAT      129 

Example,  i  lb.  of  ice  at  32°  F.  is  mixed  with  5  Ib.  of  water  at  80°  F.r 
and  the  resulting  temperature  is  48°  F.  What  is  the  latent  heat  of  ice? 

The  5  lb.  of  water  is  cooled  from  80°  F.  to  48°  F.,  that  is,  through  32°. 
Therefore  it  gave  up  5  X  32  =  IO°  B-  T-  U.  of  heat. 

This  heat  was  absorbed  by  the  ice. 

It  will  be  noticed  that  two  things  happened:  the  ice  melted  and 
the  resulting  water  was  warmed  to  48°  F. ;  that  is,  first,  the  i.  lb.  of  ice 
at  32°  F.  was  changed  to  i  lb.  of  water  at  32°  F.,  and  second,  the  i  lb. 
of  water  was  warmed  from  32°F.  to  48°  F. 

To  warm  i  lb.  of  water  from  32°  F.  to  48°  F.,  or  through  16°  F.,  requires 
16  B.T.U. 

The  total  heat  used  was  160  B.  T.  U.  Of  this,  16  B.  T.  U.  were 
used  in  warming  the  ice  water.  The  remainder  was  used  to  melt  the 
i  lb.  of  ice. 

That  is,  160—  16  or  144  B.  T.  U.  were  used  to  change  i  lb.  of  ice  at 
32°F.  to  i  lb.  of  water  at  32°  F. 

Therefore,  the  latent  heat  of  ice  is  144  B.  T.  U.  per  pound. 

Latent  heat  of  ice.  —  The  quantity  of  heat  required  to  change 
unit  weight  of  ice  to  water  without  changing  its  temperature  is 
called  the  latent  heat  of  ice. 

If  we  are  using  British  Thermal  Units  to  measure  the  quantity 
of  heat,  the  latent  heat  of  ice  is  the  number  of  B.T.U.  of 
heat  required  to  change  i  lb.  of  ice  at  32°  F.  to  i  lb.  of  water 
at  32°  F.  We  found  in  the  experiment  above  that  to  change 
i  lb.  of  ice  at  32°  F.  to  i  lb.  of  water  at  32°  F.  requires  144 
B.T.  U.  of  heat.  This  is  the  latent  heat  of  ice  when  expressed 
in  B.T.U. 

If  we  are  using  calories  to  measure  quantity  of  heat,  the 
latent  heat  of  ice  is  the  number  of  calories  required  to  change 
i  g.  of  ice  at  o°  C.  to  i  g.  of  water  at  o°  C.  To  change  i  g.  of 
ice  at  o°  C.  to  i  g.  of  water  at  o°  C.  requires  80  calories  of  heat. 
This  is  the  latent  heat  of  ice  when  expressed  in  calories. 

When  water  turns  to  ice  each  pound  or  gram  gives  up  its 
latent  heat.  When  i  lb.  of  water  at  32°  F.  turns  to  ice  at 
32°  F.,  it  gives  up  144  B.  T.  U.  of  heat,  and  when  i  g.  of  water 
at  o°  C.  turns  to  ice  at  o°  C.  it  gives  up  80  calories  of  heat. 

It  will  be  noticed  that  the  latent  heat  of  ice  expressed  in 


130 


PHYSICS    OF  THE   HOUSEHOLD 


calories  is  f  of  the  latent  heat  expressed  in  B.T. U.,  that  is, 
144  X  f  =  80.  The  reason  is,  the  degree  C.  is  -J  as  large  as  the 
degree  F.,  therefore  in  measuring  equal  changes  in  temperature 
there  are  f  as  many  degrees  C.  as  degrees  F. 

When  any  solid  is  turned  to  a  liquid,  a  certain  amount  of 
heat  is  required  to  produce  the  change.  The  amount  required 
for  i  Ib.  or  i  g.  is  the  latent  heat  of  fusion  of  the  substance. 
When  the  liquid  changes  back  to  a  solid,  this  heat  is  given  up 
again. 

Latent  Heats  of  Fusion 

Ice .  144  B.  T.  U.  per  pound 

Aluminium 140        " 

Copper 77 

Zinc 50 

Silver 39 

Experiment  10.     Latent  heat  of  steam. 

Object.  To  find  the  number  of  B.T.  U.  of  heat  given  up  wheni  Ib. 
of  steam  at  2i2°F.  changes  to  i  Ib.  of  water  at  2i2°F.,  that  is,  to  find  the 

latent  heat  of  steam. 
Method.  Weigh  an 
empty  pail  and  pour 
into  it  5  Ib.  of  ice 
water  (32°  F.). 

Place  pail  and 
water  on  a  balance 
and  pass  live  steam 
(212°  F.)  into  it  until 
Ib.  of  steam  has 
condensed  in  the 
water. 

Calculate  the  latent 
heat  of  steam. 

Example,     i  Ib.  of 
steam   at    212°  F.  is 

passed  into  20  Ib.  of  water  at  32°  F.  The  resulting  temperature  is 
86.5°  F.  What  is  the  latent  heat  of  steam? 

The  20  Ib.  of  water  is  warmed  from  32°  F.  to  86.5°  F.  or  through 
54.5°  F.,  therefore  the  water  received  20  X  54.5  =  1090  B.T.  U. 


5  Jbs.  of  water  at 
32°  F 


FIG.  90.  —  Measuring  the  latent  heat  of  steam. 


HEAT  CAPACITY,   SPECIFIC  HEAT,   LATENT  HEAT      131 

This  heat  came  from  the  steam. 

The  steam  gave  up  heat  in  two  ways.  First,  when  i  Ib.  of  steam  at 
212°  F.  changed  to  water  at  212°  F.,  second,  when  this  water  cooled  from 
212°  F.  to  86.5°  F. 

When  i  Ib.  of  water  at  212°  F.  cools  to  86.5°  F.  it  gives  up  212  —  86.5 
=  125.5  B.T.U. 

The  total  heat  given  up  by  the  steam  was  1090  B.  T.  U. ;  of  this 
125.5  B.  T.  U.  came  from  the  water  formed  by  the  steam. 

The  remainder,  1090  —  125.5.  =  964-S  B.T.U.  was  given  up  by 
i  Ib.  of  steam  in  changing  from  steam  at  212°  F.  to  water  at  212°  F. 
This  is  the  latent  heat  of  steam  per  pound. 

Note.  The  correct  value  of  the  latent  heat  of  steam  is  966  B.  T.  U. 
per  pound. 

Latent  heat  of  steam.  —  The  quantity  of  heat  required  to 
change  unit  weight  of  water  into  steam  without  changing  its 
temperature  is  called  the  latent  heat  of  steam.  In  the  experiment 
above  we  learned  that  to  change  i  Ib.  of  water  at  212°  F. 
to  i  Ib.  of  steam  at  212°  F.  requires  966  B.T.U.  of  heat. 
This  is  the  latent  heat  of  steam  when  we  measure  heat  in 
B.  T.  U. 

If  i  Ib.  of  steam  at  212°  F.  condenses  to  i  Ib.  of  water  at 
212°  F.,  it  gives  up  its  latent  heat,  namely,  966  B.T.U.  of 
heat. 

To  change  i  g.  of  water  at  100°  C.  to  i  g.  of  steam  at  100°  C. 
requires  537  calories  of  heat.  This  is  the  latent  heat  of  steam 
when  we  measure  heat  in  calories. 

If  i  g.  of  steam  at  100°  C.  condenses  to  i  g.  of  water  at  100°  C., 
it  gives  up  537  calories  of  heat. 

It  will  be  noticed  again  that  the  latent  heat  expressed  in 
calories  is  f  of  the  latent  heat  expressed  in  B.  T.  U.,  that  is, 
966x^  =  536.6,  approximately  537.  We  shall  use  the  whole 
number  537. 

When  any  substance  is  changed  from  a  liquid  to  a  vapor, 
a  certain  quantity  of  heat  is  required  to  produce  the  change. 
The  amount  required  for  i  Ib.  or  i  g.  is  the  latent  heat  of  vapor- 
ization of  the  substance.  When  the  vapor  changes  to  a  liquid 
again,  this  heat  is  given  up. 


132  PHYSICS 'OF  THE  HOUSEHOLD 

Latent  Heats  of  Vaporization. 

Water 966  B.  T.  U.  per  pound 

Alcohol 370        "         "        " 

Benzine      . 180 

Turpentine 120        "         "        " 

Latent  heat  is  work.  —  The  heat  which  disappears  when  a 
solid  is  turned  to  a  liquid  or  a  liquid  to  a  gas  does  not  affect 
the  thermometer.  Formerly  it  was  thought  that  this  heat 
was  hidden  in  some  way,  and  for  this  reason  it  was  called 
"  latent  "  heat.  We  now  know  that  when  the  heat  disappears, 
it  has  been  turned  into  work;  and  when  it  reappears,  the  work 
has  been  changed  back  into  heat.  In  other  words,  latent  heat 
is  not  heat  at  all.  It  is  work.  This  needs  some  explanation. 

Work  is  done  when  anything  is  moved  in  opposition  to  a 
force.  Now  the  molecules  of  a  solid  are  closer  together  than 
the  molecules  of  a  liquid.  When  a  solid  is  turned  to  a  liquid, 
the  molecules  are  moved  apart  in  opposition  to  their  mutual 
attraction  (cohesion).  This  requires  work,  and  the  latent  heat 
of  fusion  is  turned  into  this  work. 

Similarly,  the  molecules  of  a  vapor  are  much  farther  apart 
than  the  molecules  of  a  liquid.  The  latent  heat  of  vaporization 
is  turned  into  the  work  necessary  to  force  these  molecules  apart. 

When  the  vapor  turns  back  to  a  liquid,  and  the  liquid  to  a 
solid,  this  work  is  turned  back  again  into  heat. 

Ice  lighter  than  water.  —  In  the  paragraph  above  we  stated 
that  the  molecules  of  a  solid  are  closer  together  than  the  mole- 
cules of  a  liquid.  This  seems  to  be  contradicted  by  the  fact 
that  ice  is  lighter  than  water.  We  all  know  this  to  be  true, 
since  ice  floats  on  water,  and  water  expands  when  it  freezes. 
In  fact  i  cu.  ft.  of  water  becomes  1.09  cu.  ft.  of  ice.  How  are 
these  statements  reconciled?  We  have  all  noticed  ice  forming 
in  a  pool  or  on  a  window  pane.  How  does  it  form?  In  crys- 
tals, does  it  not  ?  Now  a  block  of  ice  is  a  mass  of  these  crystals. 
A  pound  of  iron  in  the  shape  of  nails  takes  up  more  space  than 


HEAT  CAPACITY,   SPECIFIC  HEAT,   LATENT  HEAT      133 


a  pound  of  iron  in  one  piece,  because  there  are  spaces  between 
the  nails;  so  it  is  with  the  crystals  in  a  block  of  ice.  The 
molecules  of  ice  are  closer  together  in  the  crystals  than  the 
molecules  are  in  water.  But  a  pound  of  ice  takes  up  more 
space  than  a  pound  of  water,  because  there  are  open  spaces 
between  the  crystals. 

APPLICATIONS  OF  LATENT  HEAT 

Now  that  we  know  the  meaning  of  the  terms  latent  heat 
of  ice  and  latent  heat  of  steam,  we  are  in  a  position  to  under- 
stand the  ways  in  which  man  has  turned  this  knowledge  to 
his  own  use.  For  example, 
in  the  refrigerator,  artificial 
refrigeration,  steam  heater, 
steam  cookers,  distillation, 
etc. 

The  refrigerator.  —  We 
learned  on  page  109  that 
the  refrigerator  is  a  box 
with  double  walls  between 
which  there  is  a  layer  of 
insulating  material. 


^Insulation. 
FIG.  91.  —  The  refrigerator. 


The  food  placed  in  a  re- 
frigerator is  cooled  as  fol- 
lows :  The  ice  is  placed  in 
the  upper  part  of  the  refrigerator.  The  air  in  contact  with 
it  is  cooled,  and  therefore  contracts  and  becomes  heavier, 
volume  for  volume,  than  the  remainder  of  the  air  hi  the  re- 
frigerator. This  cold,  heavy  air  sinks  down  through  an  opening 
in  the  bottom  of  the  ice  chamber,  and  forces  the  warmer, 
lighter  air  up  along  the  sides  of  the  refrigerator,  see  arrows, 
Fig.  91.  The  warm  air  comes  into  contact  with  the  ice,  is 
cooled,  contracts,  becomes  heavier,  and  sinks.  The  cold  air 
in  its  downward  passage  comes  into  contact  with  the  food  and 
absorbs  heat  from  it.  This  explains  how  the  food  is  cooled. 


134  PHYSICS  OF  THE  HOUSEHOLD 

The  cold  air  is  warmed  by  contact  with  the  food,  and  also 
by  contact  with  the  bottom  and  sides  of  the  refrigerator.  It 
thus  becomes  lighter  than  the  air  in  contact  with  the  ice,  and 
is  in  turn  forced  up  by  this  cooler,  heavier  air.  This  circulation 
of  air  is  a  convection  current.  The  convection  current  con- 
tinues as  long  as  the  bottom  and  sides  of  the  refrigerator  are 
at  a  higher  temperature  than  the  ice. 

The  motion  of  the  cold  downward  current  of  air  can  be 
illustrated  by  means  of  a  paper  spiral  supported  on  a  pin  point 
below  the  ice.  The  spiral  is  made  to  revolve  by  the  cold  down- 
ward current. 

Temperature  of  the  air  in  a  refrigerator.  —  The  temperature 
to  which  the  air  and  food  in  a  refrigerator  are  cooled  by  the 
ice  depends  upon  the  non-conducting  or  insulating  power  of 
the  walls  and  upon  the  temperature  of  the  outside  air.  The 
temperature  of  melting  ice  is  32°  F.  If  the  walls  of  the  refriger- 
ator were  perfect  insulators,  the  ice  would  absorb  heat  and  melt 
until  the  temperature  of  the  air  in  the  refrigerator  was  reduced 
to  32°  F.  Then  the  ice  would  stop  melting  and  the  air  would 
remain  at  32°  F.  That  is,  if  the  walls  were  perfect  insulators, 
the  ice  would  last  forever,  and  the  air  would  remain  forever  at 
32°  F.  There  is,  however,  no  such  thing  as  a  perfect  insulator, 
therefore  when  the  outside  air  is  above  32°  F.,  heat  passes 
into  the  refrigerator  through  the  walls.  The  amount  of  heat 
which  passes  through  the  walls  in  a  given  time  increases  as  the 
temperature  of  the  outside  air  rises,  and  decreases  as  it  falls. 
If  the  walls  are  good  insulators,  the  amount  of  heat  which  enters 
is  small ;  if  they  are  poor  insulators,  it  is  large.  Also  each  time 
the  door  of  the  refrigerator  is  opened  the  cold  air  falls  out  at 
the  bottom  and  is  replaced  by  warm  air ;  this  admits  heat  to 
the  refrigerator. 

If  the  doors  are  kept  closed  the  ice  gradually  cools  the  food, 
the  air,  and  the  inside  walls.  As  the  inside  temperature  is 
lowered,  heat  flows  in  more  rapidly  from  the  outside.  The 
temperature  finally  reached  inside  is  the  temperature  at  which 


HEAT   CAPACITY,    SPECIFIC  HEAT,   LATENT   HEAT      135 

the  heat  which  enters  through  the  walls  each  second  is  just 
equal  to  that  which  the  ice  can  absorb  each  second. 

Comparing  refrigerators.  —  We  can  measure  the  quantity  of 
heat  which  passes  through  the  walls  of  a  refrigerator  in  a  day 
by  rinding  the  weight  of  ice  melted  per  day.  We  learned  on 
page  129  that  144  B.T.  U.  of  heat  are  required  to  melt 
i  Ib.  of  ice.  Therefore,  if  20  Ibs.  o'f  ice  melt  in  the  refrigerator 
per  day,  we  know  that  20  X  144  =  2880  B.  T.  U.  of  heat 
have  passed  through  the  walls  of  the  refrigerator  in  one  day. 
If  the  water  formed  by  the  melting  ice  leaves  the  refrigerator 
at  a  temperature  of,  say,  40°  F.,  we  know  that  the  20  Ib.  of 
water  have  been  warmed  from  32°  F.  to  40°  F.,  or  through 
8°  F.  This  requires  20  X  8  =  160  B.  T.  U.  of  heat.  The 
total  amount  of  heat  which  passed  through  the  walls  in  one  day 
is  then  2880  +  160  =  3040  B.  T.  U. 

We  can  compare  two  refrigerators  as  follows.  Place  in 
each  blocks  of  ice  of  the  same  shape  and  of  the  same  weight. 
Allow  the  refrigerators  to  stand  closed  for  the  same  length  of 
time,  say  24  hr.  Find  the  temperature  of  the  water  in  each  at 
the  point  it  leaves  the  refrigerator.  At  the  end  of  the  given 
time  find  the  weight  of  ice  remaining  in  each,  and  from  this 
the  amount  of  ice  melted  in  each.  Calculate  the  amount  of 
heat  which  passed  through  the  walls  of  each  in  24  hr.  as 
above.  Other  things  being  equal,  the  better  refrigerator  is 
the  one  through  the  walls  of  which  the  lesser  amount  of  heat 
passes. 

Freezing  mixtures.  —  We  all  know  that  ice  and  salt  when 
mixed  produce  a  low  temperature.  We  have  probably  all  used 
this  mixture  to  freeze  ice  cream. 

There  are  many  substances  which  may  be  mixed  with  snow 
or  ice  to  produce  a  low  temperature.  For  example,  i  Ib.  of 
potassium  chloride  with  3  Ib.  of  snow  produces  a  temperature 
of  12°  F. ;  i  Ib.  of  ammonium  chloride  with  4  Ib.  of  snow,  5°  F. ; 
i  Ib.  of  ammonium  nitrate  with  2  Ib.  of  snow,  2°  F. ;  i  Ib.  of 
sodium  nitrate  with  2  Ib.  of  snow,  o°  F. ;  i  Ib.  of  sodium  chloride 


136  PHYSICS  OF  THE  HOUSEHOLD 

(common  salt)  with  3  Ib.  of  snow,  —  6°  F. ;  i^  lb.  of  calcium 
chloride  crystals  with  i  lb.  of  snow,  —  58°  F. 

The  lowest  temperature  produced  by  any  mixture  is  the 
freezing  point  of  the  saturated  solution  of  that  mixture.  If  an 
excess  of  the  salt  is  added  to  the  snow  or  ice,  the  low  temperature 
is  produced  more  quickly,  but  it  does  not  go  below  the  freezing 
point  of  the  saturated  solution.  If  an  excess  of  snow  or  ice  is 
added,  the  temperature  decreases  more  slowly  and  does  not  reach 
the  freezing  point  of  a  saturated  solution. 

Ice  cream  can  be  frozen  by  any  of  the  mixtures  given  above. 
A  mixture  of  salt  and  ice  is  generally  used,  because  salt  is  cheap 
and  also  because  the  mixture  produces  a  lower  temperature 
than  many  of  the  others.  The  usual  proportion  of  salt  to  ice 
is  i  of  salt  to  3  of  ice  or  snow.  If,  however,  we  wish  to  freeze 
the  ice  cream  quickly,  we  add  an  excess  of  salt.  If  we  wish  to 
freeze  it  more  slowly,  we  add  an  excess  of  ice. 

Why  do  ice  and  salt  produce  a  low  temperature  ?  When  a  block  of  ice  is 
in  a  room  which  is  at  any  temperature  above  32°  F.,  it  melts,  because  heat 
passes  from  the  room  into  the  ice.  The  rate  at  which  the  ice  melts  depends 
upon  the  rate  at  which  the  heat  moves  from  the  room  into  the  ice.  The 
heat  increases  the  velocity  of  the  ice  molecules  until  they  are  able  to  overcome 
the  attraction  of  their  fellows.  Separated  ice  molecules  are  water  molecules. 

When  we  place  salt  on  ice,  the  ice  melts  more  rapidly,  because  the  salt 
molecules  attract  the  ice  molecules.  This  attraction  enables  more  of  the 
ice  molecules  to  separate  from  their  fellows  in  a  given  time,  and  therefore 
the  melting  takes  place  more  quickly.  The  room  can  supply  heat  to  the 
ice  at  a  certain  rate.  If  the  salt  makes  the  ice  melt  at  a  more  rapid  rate, 
the  latent  heat  necessary  to  do  the  melting  comes  from  the  mixture  of  salt 
and  ice,  also  part  of  the  salt  dissolves  in  the  water  formed  by  the  melting 
ice,  and  the  heat  needed  to  change  the  solid  salt  to  a  liquid  comes  from  the 
mixture  of  salt  and  ice.  Both  of  these  causes  lower  the  temperature  of  the 
mixture  of  salt  and  ice. 

We  see  then  that  the  reason  salt  and  ice  produce  a  low  temperature  is 
that  the  salt  makes  the  ice  melt  more  rapidly  than  it  would  melt  by  the 
heat  of  the  room  alone.  The  heat  which  is  not  supplied  by  the  room  comes 
from  the  mixture  of  salt  and  ice  itself,  and  therefore  the  temperature  of  the 
mixture  is  lowered.  The  lowest  temperature  produced  is  the  freezing  point 
of  a  concentrated  solution  of  salt  and  water,  —  6°  F. 


HEAT  CAPACITY,   SPECIFIC   HEAT,   LATENT  HEAT      137 

When  the  mixture  of  salt  and  ice  is  used  to  freeze  ice  cream,  part  of  the 
latent  heat  necessary  to  change  the  ice  and  salt  to  the  liquid  condition 
is  taken  from  the  ice  cream  and  the  ice  cream  is  frozen. 

Artificial  ice  and  artificial  cooling.  —  We  know  that  when 
steam  is  condensed  to  water  it  gives  up  its  latent  heat,  and  when 
it  is  again  changed  from  water  to  steam  it  takes  up  this  latent 
heat. 

This  is  true  of  all  gases,  and  is  the  principle  upon  which  the 
successful  artificial  ice  machines  are  built.  The  gas  in  most 
common  use  is  ammonia.  It  is  used  because  it  is  easily 
condensed  to  liquid  ammonia,  and  liquid  ammonia  boils  at 
a  very  low  temperature,  —  29°  F.,  when  the  pressure  is 
i  atmosphere. 

The  artificial  ice  machine,  Fig.  92,  is  designed  to  cool  cold 
storage  rooms,  or  to  make  artificial  ice,  as  desired.  It  consists 
of  a  compressor  (^4),  two  sets  of  coils  (B)  and  (C),  a  valve  for 
expanding  the  gas  to  lower  pressure,  and  the  ice  tank  (D) 
containing  brine,  usually  calcium  chloride  dissolved  in  water. 
The  coils  of  pipe  are  charged  with  liquid  ammonia  free  from 
water. 

The  operation  of  the  machine  is  as  follows :  The  compressor 
is  driven  by  some  form  of  power  appliance,  such  as  a  steam 
engine,  gas  engine,  etc.  It  pumps  ammonia  gas  from  the  coils 
(C)  and  forces  it  into  the  coils  (B)  at  a  pressure  of  about  135  Ib. 
per  square  inch  above  atmospheric  pressure.  Under  this  pressure 
the  ammonia  gas  is  turned  to  a  liquid,  and  in  turning  to  a  liquid 
gives  up  its  latent  heat.  This  heat  is  absorbed  by  a  stream  of 
cold  water  which  is  kept  flowing  through  the  condenser  (B). 
The  expansion  valve  is  simply  a  valve  with  a  very  small  open- 
ing, through  which  the  liquid  ammonia  passes  in  a  small  stream 
and  enters  the  coils  in  (C).  In  (C)  the  pressure  is  kept  at 
about  1 6  Ib.  per  square  inch  above  atmospheric  pressure.  Here 
the  liquid  ammonia  turns  to  a  gas,  and  in  doing  so  absorbs  its 
latent  heat  again.  It  takes  this  heat  from  the  brine  which  is 
kept  flowing  through  the  evaporator.  It  thus  lowers  the  tem- 


138 


PHYSICS  OF  THE  HOUSEHOLD 


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HEAT   CAPACITY,    SPECIFIC  HEAT,   LATENT  HEAT      139 


perature  of  the  brine.  At  a  gauge  pressure  of  16  Ib.  ammonia 
liquid  boils,  that  is,  turns  to  a  gas,  at  a  temperature  of  o°  F. 
The  brine  is  cooled  to  about  20°  F. 

The  ammonia  gas  is  then  pumped  again  into  (B)  and  the 
whole  operation  is  repeated  continuously. 

When  it  is  desired  to  cool  a  cold  storage  room  in  a  house, 
hotel,  steamship,  or  cold  storage  warehouse,  the  brine  is  pumped 
from  the  ice  tank  to  coils  of  pipe  placed  near  the  ceiling  of  the 
room.  The  brine  is  kept  circulating  between  the  tank  and  the 
coils,  and  cools  the  cold 
storage  room  precisely  as 
a  block  of  ice  cools  a  re- 
frigerator. 

When  it  is  desired  to 
make  artificial  ice,  long 
narrow  cans  filled  with 
pure  water  are  placed  in 
the  ice  tank,  as  shown  in 
the  figure.  The  cold  brine 
freezes  the  water  to  a  solid 
block  in  from  40  to  48  hr. 

Steam  heating  system. — 
The  steam  heating  system 
of  a  house  is  arranged  as 
shown  in  Fig.  93.  The  fire 
under  the  boiler  boils  the 
water  and  generates  steam. 
This  steam  passes  up  the 
steam  pipe  and  into  the 

radiator;  here  it  condenses  to  water  and  gives  up  its  latent 
heat  to  the  air  in  the  room.  The  water  flows  back  through 
the  steam  pipe  to  the  boiler,  is  again  turned  to  steam,  and  so 
on.  The  operation  is  repeated  as  long  as  there  is  sufficient  fire 
under  the  boiler.  Each  pound  of  steam  that  condenses  in  the 
radiator  gives  up  its  latent  heat,  966  B.  T.  U.  of  heat.  In 


FIG.  93.  —  A  steam  heating  system. 


140  PHYSICS  OF  THE  HOUSEHOLD 

addition,  if  the  water  leaves  the  radiator  at  a  temperature  of, 
say,  190°  F.,  each  pound  of  steam  water  cools  from  212°  to  190°, 
or  22°  F.,  and  in  doing  so  gives  up  22  B.  T.  U.  of  heat.  Under 
these  circumstances  the  total  amount  of  heat  given  up  by 
each  pound  of  steam  is  966  +  22  =  988  B.  T.  U. 

Steam  cookers.  —  Food  is  cooked  in  a  steam  cooker  as  fol- 
lows. Water  is  boiled  in  the  bottom  of  the  cooker  and  the 
steam  formed  passes  up  into  the  divisions  above.  Here  the 
steam  heats  the  food,  partly  by  convection  and  radiation,  and 
partly  by  condensation.  A  pound  of  steam  in  condensing  gives 
966  B.  T.  U.  of  heat  to  the  food.  The  food  is  cooked  at  a  tem- 
perature never  above  212°  F. 

Distillation.  —  Pure  water  can  be  obtained  from  impure 
water  by  distillation.  The  impure  water  is  boiled  and  the 


FIG.  94.  —  Distilling  apparatus. 

steam  formed  is  condensed  to  water  again.  The  condensed 
steam  is  pure  water.  One  form  of  distilling  apparatus  is  shown 
in  Fig.  94.  The  impure  water  is  boiled  in  a  flask,  and  the 
resulting  steam  is  condensed  in  the  condenser.  The  pure  water 
is  caught  in  a  flask  at  the  lower  end  of  the  condenser.  The 
condenser  consists  of  a  tube  with  a  water  jacket;  cold  water 
is  made  to  enter  the  water  jacket  at  the  bottom  and  leave  it 


HEAT  CAPACITY,   SPECIFIC  HEAT,   LATENT  HEAT      141 

at  the  top.  This  cold  water  condenses  the  steam.  When  the 
impure  water  is  boiled,  the  nonvolatile  impurities  are  left  in 
the  flask  and  the  steam  is  pure  water.  If  the  impurities 
are  volatile,  they  cannot  be  separated  from  the  water  by 
this  process,  because  they  vaporize  and  pass  over  with  the 
steam. 

Domestic  distilling  apparatus.  —  A  common  form  of  domestic 
still  is  shown  in  Fig.  95.  It  consists  of  three  sections  which 
fit  one  into  the  other.  The  water  is  boiled 
in  the  bottom  section,  the  distilled  water  is 
caught  in  the  middle  section,  and  a  reserve 
supply  of  cold  water  to  be  distilled  is  kept 
in  the  upper  section.  A  pipe  passes  up 
through  the  bottom  of  the  middle  section. 
The  water  is  distilled  as  follows :  The  steam 
formed  by  the  boiling  water  in  the  lower 
section  passes  through  the  pipe  of  the  middle 
section  and  strikes  against  the  cold  bottom  FlG"  9S'  ~£  domestic 
of  the  upper  section.  It  is  here  condensed 
to  water  and  falls  into  the  middle  section.  This  is  the  distilled 
water  which  accumulates  in  the  middle  section. 


EXERCISES 

1.  Which  would    make    the    better  foot  warmer,  5  Ib.  of  water  at 
212°  F.  or  5  Ib.  of  iron  at  300°  F.  ?     Assume  that  they  both  cool  to  80°  F. 
How  much  heat  does  each  give  up? 

2.  How  many  B.  T.  U.  are  given  up  when  5  Ib.  of  each  of  the  fol- 
lowing cool  10°  F. ;  —  air,  alcohol,  glass,  iron,  silver? 

3.  How  many  B.T.  U.  are  required   to   heat  10  Ib.  of   each  of  the 
following  20°  F.  —  water,  brass,  glass,  ice,  mercury? 

4.  If  a  room  is  24  X  12  X  10  ft.,  how  many  pounds  of  air  are  there 
in  it?     (i  cu.  ft.  of   air  weighs  i^  oz.)     How  many  B.T.  U.   will   this 
air  give  up  if  it  cools  from  70°  F.  to  40°  F.  ? 

5.  In  a  hot- water   heating   system  the  water  enters  the  radiator  at 
180°  F.,  and  leaves  it  at  110°  F.     How  many  B.  T.  U.  does  each  pound 
of  water  give  to  the  room? 


142  PHYSICS  OF  THE  HOUSEHOLD 

6.  In  a  hot-air  furnace  the  air  enters  the  furnace  at  20°  F.,  and  leaves 
it  at  180°  F.     How  many  B.T.  U.  does  each  pound  of  air  receive  in  the 
furnace  ? 

7.  A  hot- water  bottle  has  5  Ib.  of  water  in  it  at  180°  F.  at  night.     In 
the  morning  the  temperature  is  70°  F.     How  many  heat  units  have 
escaped? 

8.  A  carriage  foot  warmer  made  of  brick  weighs  6  Ib.,  its  tempera- 
ture is  400°  F.     How  much  heat  does  it  give  up  in  cooling  to  60°  F.  ? 

9.  Define  latent  heat  of  fusion,  latent  heat  of  vaporization. 

10.  How  many  heat  units  are  required  to  melt  10  Ib.  of  ice  and  warm 
it  up  to  82°  F.? 

11.  Four  Ib.  of  ice  at  32°  F.  when  placed  in  5  Ib.  of  water  at  150°  F. 
melts  and  lowers  the  temperature  to  62°  F.     Calculate  the  latent  heat 
of  ice  from  this  experiment. 

12.  How  many  heat  units  are  given  up  when  10  Ib.  of  steam  con- 
denses in  a  radiator  ? 

13.  When  potatoes  are  being  boiled  in  water,  what  is  the  temperature 
of  the  water?     Can  this  temperature  be  increased  by  turning  on  more 
gas?     What  is  the  extra  heat  doing?     Is  it  a  waste  of  gas  to  boil  things 
vigorously  ? 

14.  When  vegetables  are  kept  in  a  cellar,  sometimes  a  tub  of  water 
is  placed  in  the  cellar  to  prevent  them  from  freezing.     If  the  tub  holds 
200  Ib.  of  water,  how  many  B.  T.  U.  will  it  give  out  in  changing  from 
water  at  32°  F.  to  ice  at  32°  F.? 

(Note.     The  juices  in  vegetables  are  solutions,  and  solutions  freeze 
at  temperatures  below  32°F.) 

15.  There  is  50  Ib.  of  ice  in  a  refrigerator.     If  400  B.  T.  U.  leak  through 
the  sides  of  the  refrigerator  per  hour,  how  long  will  the  ice  last? 

16.  If  50  Ib.  of  ice  lasts  in  a  refrigerator  20  hr.,  how  many  B.T.  U. 
leak  through  the  sides  in  one  hour? 

17.  A  room  is  warmed  by  a  steam  radiator.     If  5  Ib.  of  steam  con- 
denses in  the  radiator  every  hour,  how  much  heat  does  the  room  receive 
each  hour? 

18.  A  pound  of  coal  gives  up  15,000  B.  T.  U.  when  burned.     How 
many  pounds  of  boiling  water  could  be  turned  to  steam  by  the  heat 
from  i  Ib.  of  coal? 

19.  Explain  the  statement,  "  Latent  Heat  is  Work." 

20.  If  30  Ib.  of  ice  melts  in  one  day  in  a  refrigerator,  how  many  heat 
units  pass  through  the  walls  of  the  refrigerator  per  hour? 

21.  Make  a  drawing  showing  how  the  ice  cools  the  air  in  a  refrigerator. 

22.  Explain   the  operation  of  an  artificial  ice  machine.     Make  a 
drawing. 


HEAT  CAPACITY,   SPECIFIC  HEAT,   LATENT   HEAT      143 

23.  Make  a  drawing  illustrating  the  steam  heating  system.     Explain 
the  operation  of  the  system. 

24.  If   4  Ib.  of  steam    at    212°  F.  enters  a   radiator  and  leaves  as 
water  at  180°  F.,  how  many  heat  units  has  it  given  up? 

25.  If  4  Ib.  of  steam  condenses  in  a  radiator,  how  many  heat  units 
does  it  give  up  ? 

26.  Explain  how  the  food  is  cooked  in  a  steam  cooker. 

27.  Make  a  drawing  of  the  distillation  apparatus,  and  explain  how 
the  water  is  purified. 

28.  Describe  a  domestic  still. 

29.  State  some  other  ways  in  which  man  makes  use  of  latent  heat. 


CHAPTER  XII 
EVAPORATION,   DEW  POINT,   BOILING   POINT 

Evaporation.  —  We  are  all  familiar  with  many  examples  of 
evaporation.  For  instance,  when  a  floor  dries  after  being 
scrubbed,  the  water  leaves  by  evaporation;  when  dishes  are 
washed  and  dried,  the  thin  film  of  water  remaining  on  each  dish 
leaves  by  evaporation ;  when  clothes  are  hung  on  a  line  to  dry, 
the  water  leaves  by  evaporation ;  etc.  Let  us  now  learn  why 
liquids  evaporate. 

Our  theory  regarding  the  constitution  of  matter  and  our 
theory  regarding  the  nature  of  heat  give  us  an  explanation  of 
evaporation.  These  theories  assume  that  all  liquids  are  com- 
posed of  molecules  which  are  held  together  by  their  mutual 
attraction.  When  the  liquid  is  warmed  above  —273°  C.,  the 
molecules  are  in  constant  and  rapid  motion,  and  as  a  result 
there  are  spaces  between  them.  If  a  liquid  could  be  very  much 
enlarged,  it  would  look  something  like  a  swarm  of  gnats,  the 
molecules  (the  gnats)  being  in  very  rapid  motion.  The  aver- 
age velocity  of  the  molecules  is  constant  at  any  given  tempera- 
ture, but  the  individual  velocities  of  the  molecules  vary  greatly 
and  change  from  instant  to  instant.  This  variation  is  due  to 
the  frequent  impact  between  molecules. 

The  explanation  of  evaporation  is  as  follows :  Molecules 
which  are  near  the  surface  of  the  liquid  and  which  have  a  high 
velocity  escape  from  it.  Some  are  drawn  back  into  the  liquid 
by  the  attraction  of  the  surface  molecules,  but  some  escape 
into  the  air  above,  and  gradually  all  escape  in  this  way. 

The  explanation  of  evaporation,  then,  is  that  molecules 

144 


EVAPORATION,   DEW  POINT,   BOILING   POINT         145 

are  near  the  surface  and  which  have  a  great  velocity  escape 
from  the  liquid  into  the  space  above. 

Cooling  by  evaporation.  —  We  are  all  familiar  with  the  cool- 
ing effect  produced  by  evaporation.  For  example :  the  evapo- 
ration of  water  sprinkled  on  the  floor  cools  the  air  in  the  room ; 
the  evaporation  of  water  from  the  ground  after  a  rain  cools  the 
air  outdoors ;  the  evaporation  of  perspiration  cools  our  bodies. 

The  explanation  of  this  cooling  effect  is  as  follows :  When  a 
liquid  turns  to  a  vapor  in  any  way,  it  absorbs  its  latent  heat 
from  objects  near  it  and  cools  them.  This  heat  passes  into  the 
water  and  turns  into  work;  it  turns  into  the  work  necessary 
to  separate  the  molecules  against  the  force  of  cohesion. 

Air  saturated  with  water  vapor.  —  If  a  jar  filled  half  full  of 
water  is  left  open,  all  the  water  evaporates  in  time.  If,  how- 
ever, the  jar  is  closed,  the  water  does  not  evaporate.  In  the 
open  jar,  water  molecules  escape  from  the  surface  of  the 
liquid  and  pass  into  the  air  above.  When  in  the  air  they  move 
about  freely,  just  as  air  molecules  do ;  they  collide  with  each 
other,  and  with  air  molecules.  In  this  irregular  motion  some 
of  the  water  molecules  strike  back  into  the  liquid  again,  but 
some  pass  out  of  the  jar  and  are  lost.  Gradually  all  of  the 
water  molecules  pass  out  of  the  jar  in  this  way,  that  is,  the 
liquid  evaporates. 

In  the  closed  jar,  water  molecules  leave  the  liquid  and 
pass  into  the  air,  but  none  escape  from  the  jar.  The  water 
molecules  in  the  air  move  about  as  above,  and  some  strike  back 
into  the  liquid.  In  time  the  air  is  so  filled  with  vibrating  water 
molecules  that  the  number  that  go  back  into  the  liquid  in  one 
second  is  just  equal  to  the  number  which  leave  the  liquid  in  one 
second.  When  the  air  is  in  this  condition  it  is  said  to  be  satu- 
rated with  water  vapor. 

The  number  of  water  molecules  required  to  saturate -a  given 
volume  of  air  increases  with  the  temperature,  or,  in  other  words, 
a  given  volume  of  warm  saturated  air  contains  more  water 
vapor  than  the  same  volume  of  cold  saturated  air.  If  warm 

L 


146  PHYSICS  OF  THE  HOUSEHOLD 

saturated  air  is  cooled,  a  certain  amount  of  the  water  vapor 
condenses  to  liquid  water,  and  the  air  remains  saturated  at 
the  lower  temperature. 

Clothes  drying  on  a  line.  —  Let  us  now  examine  carefully 
one  instance  of  evaporation.  When  we  understand  this,  we 
shall  have  a  deeper  insight  into  all  cases  of  evaporation.  Let 
us  consider  the  case  of  clothes  drying  on  a  line. 

Let  us  answer  the  question,  "  Why  do  clothes  dry  more  rapidly 
on  a  windy  day  than  on  a  day  when  the  air  is  still?  "  When 
the  wet  clothes  are  hung  out  on  a  line  on  a  still  day  the  water 
molecules  pass  from  the  clothes  to  the  air  near  the  clothes.  This 
air  soon  becomes  nearly  saturated,  and  many  of  the  water  mole- 
cules pass  back  from  the  air  to  the  clothes  again.  Some,  of  course, 
penetrate  into  the  outer  air,  and  are  lost,  and  gradually  all  do 
so.  In  this  way  the  clothes  dry  slowly.  When  the  wet  clothes 
are  hung  out  to  dry  on  a  windy  day,  however,  the  water  mole- 
cules escape  into  the  air,  as  above,  but  since  the  air  is  in  motion 
the  water  molecules  are  carried  away  with  the  air,  and  few  of 
them  strike  back  into  the  clothes.  In  this  way  the  clothes  are 
dried  rapidly.  This  explains  why  clothes  dry  more  rapidly  on 
a  windy  day  than  on  a  still  day. 

Let  us  now  answer  the  question,  "  Why  do  clothes  dry  more 
rapidly  when  the  air  is  dry  than  when  it  is  moist?  "  When  the 
air  is  dry,  water  molecules  pass  from  the  clothes  to  the  dry  air, 
and  a  few  pass  back  from  the  air  to  the  clothes.  When  the  air  is 
moist,  however,  just  as  many  water  molecules  pass/row  the  clothes 
to  the  air,  but  a  greater  number  pass  from  the  air  to  the  clothes, 
and  therefore  the  clothes  dry  more  slowly.  If  the  air  is  satu- 
rated with  water  vapor,  as  it  is  when  there  is  a  heavy  fog,  or  at 
times  just  before  a  rain,  the  clothes  do  not  dry  at  all.  In  this 
case,  at  a  given  temperature,  just  as  many  water  molecules 
leave  the  clothes  per  second  as  when  the  air  is  dry,  but  the 
number  of  molecules  which  pass  from  the  air  to  the  clothes  is 
equal  to  the  number  passing  from  the  clothes  to  the  air.  The 
result  is  the  clothes  do  not  dry. 


EVAPORATION,   DEW   POINT,   BOILING  POINT         147 

What  has  been  said  of  evaporation  in  connection  with  the 
drying  of  clothes  is  true  of  evaporation  in  general. 

Evaporation  in  nature.  —  The  process  of  evaporation  is  of 
great  importance  in  nature.  All  the  moisture  which  falls  upon 
the  earth  in  the  form  of  rain,  snow,  hail,  or  dew  is  water  which 
passed  into  the  air  by  evaporation.  The  heat  of  the  sun  causes 
water  to  evaporate  from  the  surface  of  the  ground  and  from  the 
surface  of  oceans,  lakes,  and  rivers.  This  water  vapor  enters 
the  air  and  moves  with  it  over  the  earth.  When  the  air  is 
cooled  it  becomes  saturated,  and  on  further  cooling  the  moisture 
separates  in  the  form  of  liquid  drops  in  cloud,  fog,  rain,  hail, 
snow,  or  dew. 

Cloud,  rain,  hail,  and  snow.  —  If  the  air  at  some  distance 
above  the  earth's  surface  is  cooled  somewhat  below  the  tem- 
perature of  saturation,  the  water  forms  minute  drops  about 
dust  particles  in  the  air.  These  minute  drops  constitute 
clouds.  If  the  air  is  cooled  somewhat  more,  the  drops  increase 
in  size  and  fall  as  rain.  If  the  rain  in  falling  to  the  earth  passes 
through  a  layer  of  air  at  a  temperature  below  freezing,  the  rain 
freezes  and  becomes  hail.  If  the  water  vapor  condenses  from 
the  air  at  a  temperature  below  freezing,  it  forms  snow  crystals 
and  falls  as  snow. 

Dew  and  fog.  —  When  the  sun  goes  down  at  night,  the  earth 
cools  by  radiation.  Heat  radiates  more  rapidly  from  earth, 
grass,  leaves,  etc.,  than  it  does  from  air,  and  as  a  result  these 
may  be  quite  cool  while  the  air  is  still  warm.  When  warm, 
moist  air  comes  in  contact  with  the  cold  grass,  leaves,  .etc., 
it  is  cooled,  and  if  the  temperature  to  which  it  is  cooled  is  be- 
low the  temperature  at  which  the  air  is  saturated,  moisture/is 
deposited  on  the  grass,  leaves,  etc.  This  moisture  is  dew.  If 
the  air  itself  cools  below  the  temperature  of  saturation,  the 
water  condenses  as  minute  drops  on  dust  particles  in  the  air. 
These  minute  drops  constitute  fog.  A  fog  is  simply  a  cloud 
formed  at  the  surface  of  the  earth,  instead  of  above  it.  Fogs 
at  sea  are  formed  when  a  warm  moisture-laden  current  of  air 


148  PHYSICS  OF  THE  HOUSEHOLD 

is  cooled,  either  by  meeting  a  cold  current  of  air,  or  by 
radiation. 

Dew  point  and  relative  humidity.  —  The  dew  point  of  air  is 
the  temperature  at  which  the  air  is  saturated  and  dew  deposits. 
The  dew  point  varies  of  course  with  the  amount  of  moisture 
in  the  air.  If  the  air  is  nearly  saturated,  the  dew  point  is  near 
the  temperature  of  the  air.  If  the  air  is  very  dry,  the  dew  point 
is  a  temperature  much  below  that  of  the  air.  The. relative 
humidity  of  the  air  at  any  time  is  the  ratio  of  the  quantity  of 
moisture  it  contains  per  unit  volume,  to  the  quantity  it  could 
contain  per  unit  volume  at  that  temperature.  For  example, 
air  at  any  temperature  has  a  relative  humidity  of  50  per  cent 
when  it  contains  one  half  as  much  moisture  as  it  could  contain 
at  this  temperature. 

Boiling  point.  —  If  'we  place  a  thermometer  in  a  liquid  and 
then  heat  the  liquid,  we  find  that  the  liquid  boils  at  a  certain 
temperature.  If  we  place  two  burners  under  the  liquid  to  in- 
crease the  amount  of  heat  supplied,  we  find  that  the  liquid  still 
boils  at  the  same  temperature.  This  temperature  is  the  boil- 
ing point  of  the  liquid.  The  normal  boiling  point  of  any  liquid 
is  the  highest  temperature  to  which  it  can  be  heated  when 
under  a  pressure  of  one  atmosphere. 

This  is  one  definition  of  the  boiling  point  of  a  liquid.  It 
can  be  defined  in  other  ways.  If  we  watch  a  liquid  boiling 
in  a  glass  vessel,  we  notice  that  bubbles  form  at  the  bottom  and 
rise  to  the  top,  increasing  in  size  as  they  rise.  Since  the  bubbles 
rise  to  the  top,  it  is  evident  that  the  pressure  of  the  vapor  pass- 
ing from  the  liquid  into  the  bubble  is  equal  to  the  pressure  above 
the  liquid.  The  normal  boiling  point  of  a  liquid  then  may  be 
defined  as  the  temperature  at  which  the  pressure  of  the  vapor 
passing  from  the  liquid  is  equal  to  one  atmosphere. 

Boiling  point  varies  with  the  pressure.  —  It  can  be  shown 
that  a  liquid  boils  at  a  higher  temperature  when  the  pressure 
upon  it  is  increased,  as  follows :  Arrange  a  flask  as  shown  in 
Fig.  96.  The  flask  contains  water  and  is  fitted  with  a  rubber 


EVAPORATION,   DEW   POINT,    BOILING   POINT         149 


under  increased  pres- 
sure. The  mercury 
column  a,  /3  gives  the 
increased  pressure. 


stopper  with  two  holes.  In  one  is  placed  a  thermometer,  and 
in  the  other  a  bent  tube  C  which  is  tapered  slightly  at  the  lower 
end  and  cut  off  in  a  slanting  direction.  By 
pouring  successive  quantities  of  mercury 
into  the  cylinder  D  the  pressure  is  in- 
creased. It  will  be  found  that  the  water 
boils  at  higher  temperatures  as  the  pres- 
sure upon  it  is  increased. 

It  can  be  shown  that  a  liquid  boils  at 
a  lower  temperature  when  the  pressure  on 
it  is  decreased,  as  follows :  Arrange  a  flask 
as  shown  in  Fig.  97.  Boil  water  in  the 
flask;  then  remove  it  from  the  fire  and  FlG-  96.  — To  find  the 

boiling  point  of  water 

attach   the    bent    tube   to   an   air   pump. 

Start  pumping  the  air  and  steam  out  of 

the  flask   and   observe   the   thermometer. 

It   .will    be    found    that    the   water    boils 

at  a  low  temperature  when  the  pressure  on  it  is  decreased. 
This  can  be  shown  also  by  means  of  the  apparatus  illustrated 
in  Fig.  98.  Proceed  as  follows.  Boil  water 
in  a  round-bottomed  flask,  then  remove  it 
from  the  fire,  and  close  it  air-tight.  The 
water  will  cease  to  boil  when  removed  from 
the  fire.  If  now  the  flask  is  inverted  and  cold 
water  is  poured  on  the  bottom,  the  water  will 
begin  to  boil  vigorously,  although  it  is  being 
cooled.  The  cold  water  condenses  the  water 
vapor  in  the  flask,  and  thereby  decreases  the 
pressure  on  the  water.  The  water  is  at  a 

FIG.  97.— To  boil  temperature  below  its  normal  boiling  point,  but 
^  ^°^s  Decause  ^ts  boiling  point  under  reduced 
pressure  is  lower  than  the  normal  boiling  point. 
Why  change  of  pressure  affects  the  boiling  point  of  a  liquid.  — 

When  a  liquid  boils,  the  molecules  which  escape  from  it  are 

those  which  have  sufficient  velocity  to  overcome  the  attrac- 


PHYSICS   OF  THE  HOUSEHOLD 


tion  of  the  other  molecules,  and  to  overcome  the  pressure  of  thft 
air  or  vapor.  If  the  pressure  is  increased,  the  molecules  which 
escape  must  have  a  still  greater  velocity  to  overcome  the  greater 
pressure,  that  is,  they  must  have  a 
higher  temperature.  When  the  pres- 
sure is  decreased,  however,  the  mol- 
ecules are  able  to  escape,  when  they 
have  a  smaller  velocity,  that  is,  when 
they  are  cooler.  This  explains  why 
liquids  boil  at  a  temperature  above 
the  normal  boiling  point  when  the 
pressure  on  them  is  increased,  and 
why  they  boil  at  a  temperature  below 
the  normal  boiling  point  when  the 
pressure  on  them  is  decreased. 

Difference  between  evaporation  and 
boiling.  —  When  a  liquid  is  open  to 
the  atmosphere,  vapor  forms  at  its 
surface.  This  is  evaporation.  If  its 
temperature  is  increased,  the  rate  at  which  vapor  forms  in- 
creases, and  when  the  boiling  point  is  reached,  vapor  forms  at 
the  surface  of  the  liquid  and  also  at  the  surface  of  the  bubbles 
formed  in  the  liquid.  This  is  boiling.  The  difference  between 
evaporation  and  boiling,  then,  is  as  follows :  first,  when  a  liquid 
evaporates,  vapor  forms  only  at  the  surface  of  the  liquid,  but 
when  it  boils,  vapor  forms  at  the  surface  of  the  liquid  and 
also  at  the  surface  of  the  bubbles  formed  in  the  liquid ;  second, 
evaporation  takes  place  at  all  temperatures  below  the  boiling 
point,  but  boiling  takes  place  at  only  one  temperature  under  a 
given  pressure. 

Applications  of  boiling.  —  Two  of  the  commonest  methods  of 
cooking  food  are  by  boiling  and  by  stewing.  Food  is  said  to 
be  boiled  when  it  is  cooked  in  boiling  water.  It  is  said  to  be 
stewed  when  it  is  cooked  in  water  at  a  temperature  below  the 
boiling  point.  When  an  egg  is  boiled  in  water  it  is  cooked  at 


EVAPORATION,   DEW  POINT,   BOILING  POINT         151 


a  temperature  never  above  the  boiling  point,  namely,  212°  F. 
Also,  when  food  is  cooked  in  a  double  boiler,  Fig.  99,  the  tem- 
perature at  which  it  is  cooked  is  never  above  the  boiling  point 
of    the  water  in   the  outer 
vessel,  212°  F.     Pure  water 
boils  at   212°  F.,  no  matter 
how  fiercely  the  fire  is  burn- 
ing ;  therefore  in  boiling  food 
it  is  economy   to   use  a  fire 
which  keeps  the  water  just 
boiling.    Any  greater  amount 
of  fire  is  wasted  in  boiling 
away  the  water.  FlG-  "-~ The  double  boUer-    The  food 

•L  in  a  is  cooked  by  heat  received  from 

When  water  has  any  sub-          the  water  c  and  the  steam  b. 

stance  dissolved  in  it,  it  boils 

at  a  temperature  somewhat  above  212°  F.  When  vegetables, 
fruits,  meats,  etc.,  are  boiled  in  water,  some  of  their  constituents 
dissolve  in  the  water,  and  therefore  the  boiling  point  is  slightly 
above  212°  F. 


EXERCISES 

1.  Explain  why  liquids  evaporate. 

2.  Explain  why  evaporating  water  cools  surrounding  objects. 

3.  When  is  air  said  to  be  saturated  with  water  vapor? 

4.  Explain  why  clothes  dry  more  rapidly  on  a  windy  day  than  on  a 
still  day. 

5.  Explain  why  clothes  dry  more  rapidly  on  a  dry  day  than  on  a 
moist  day. 

6.  Explain  how  clouds,  rain,  hail,  and  snow  are  formed. 

7.  Define  dew  point  and  relative  humidity. 

8.  What  is  the  boiling  point  of  a  liquid? 

9.  Explain  why  a  change  in  the  pressure  upon  water  changes  its 
boiling  point. 

10.  What  are  the  differences  between  evaporation  and  boiling? 

11.  What  is  the  advantage  of  a  double  boiler? 


CHAPTER  XIII 


SOURCES   OF   HEAT.     HEAT  AND   WORK 

Fuels.  —  The  common  sources  of  artificial  heat  are  wood, 
coal,  oil,  gas,  alcohol,  coke,  charcoal,  and  the  electric  current. 
The  amount  of  heat  obtained  from  some  of  these  sources  is 
approximately 1  as  follows : 

Heat  per  Pound  of  Fuel 

r>.  1.  U. 

Anthracite  coal  (high  grade) 14,500 

Bituminous  coal  (high  grade) 15,000 

Wood  (air  dried) 7,5°° 

Petroleum 20,000 

Gasoline 21,000 

Alcohol 12,000 

Heat  from  Gas  per  Cubic  Foot  and  per  Pound 


GAS 

B.  T.  U. 

PER  CUBIC  FOOT 

B.  T.  U. 
PER  POUND 

Natural  gas      

I,OOO 

22  OOO 

Coal  gas 

7OO 

2  1  ,OOO 

Water  gas 

•3  2O 

7  OOO 

Producer  gas    . 

3«%l 

I4.O 

2  IOO 

Heat  from  electricity.  —  One  kilowatt  hour  =  3412  B.  T.  U. 
This  is  about  equal  to  the  heat  produced  when  \  Ib.  of  coal  is 
burned. 

1  The  figures  given  in  these  tables  are  necessarily  approximations  because  fuels 
vary  in  composition. 

152 


Gasoline %** 

.5  lb.  per  Imperial  gallon 


SOURCES  OF  HEAT.      HEAT  AND   WORK  153 

Weights  of  fuels. - 

Coal 2000  lb.  per  ton 

Hardwood 4000  lb.  per  cord 

Pine 2000  lb.  per  cord 

f  6.5  lb.  per  U.  S.  gallon 

Petroleum .6. 

8  lb.  per  Imperial  gallon 

5  lb.  per  U.  S.  gallon 
5  lb.  per  Imperial  gal 

[  6.8  lb.  per  U.  S.  gallon 

Alcohol  90  per  cent -in 

(  8.3  lb.  per  Imperial  gallon 

Natural  gas 45.5  lb.  per  1000  cu.  ft. 

Coal  gas 32  lb.  per  1000  cu.  ft. 

Water  gas 45.5  lb.  per  1000  cu.  ft. 

Producer  gas 65.5  lb.  per  1000  cu.  ft. 

Comparison  of  fuels.  —  When  we  know  the  prices  of  the 
different  fuels,  we  can  use  the  tables  above  to  compare  the 
amounts  of  heat  purchased  for  $i.  For  example : 

Bituminous  coal  at  $4  per  ton  gives  500  X  15,000  =  7,500,000 
B.T.U.  for$i. 

Hard  wood  at  $8  per  cord  gives  500  X  7500  =  3,750,000 
B.T.U.  for  $i. 

Petroleum  at  20  ct.  per  U.  S.  gallon  gives  6J  X  5  X  20,000 
=  650,000  B.T.U.  for  $i. 

Gasoline  at  20  ct.  per  U.  S.  gallon  gives  5^  X  5  X  21,000 
=  577,5oo  B.T.U.  for  $i. 

Natural  gas  at  $i  per  1000  cu.  ft.  gives  1,000,000  B.  T.  U. 
for$i. 

Electricity  at  5  ct.  per  kilowatt  hour  gives  3412  X  20 
=  68,240  B.T.U.  for  $i. 

At  the  prices  quoted  above  it  will  be  noticed  that  for  $i 
coal  gives  yj  times  as  much  heat  as  natural  gas,  and  about 
no  times  as  much  heat  as  electricity. 

Gas  and  electricity  have  certain  advantages,  such  as  con- 
venience, cleanliness,  etc.,  which  offset  to  some  extent  the 


154  PHYSICS  OF  THE  HOUSEHOLD 

greater  cost  of  the  heat  supplied  by  them,  but  coal  is  much  the 
cheapest  source  of  heat. 

HEAT  AND  WORK 

Heat  engines.  —  The  human  race  has  made  great  progress  in 
the  last  hundred  years,  and  this  progress  has  been  made 
possible  largely  by  the  steam  engine.  In  steam  engines,  part 
of  the  heat  produced  by  burning  fuel  is  turned  into  work.  These 
engines  do  work  which  was  formerly  done  by  human  beings  or 
by  animals,  or  not  done  at  all.  In  this  way  the  human  race 
has  been  freed  from  heavy  drudgery,  and  the  energy  thus  saved 
is  devoted  to  higher  forms  of  work. 

Steam  engines  are  called  heat  engines  because  in  them  heat 
is  turned  into  work.  Another  form  of  heat  engine  is  the  gas  or 
gasoline  engine.  Let  us  now  study  these  two  types  of  heat 
engine,  and  learn  how  they  turn  heat  into  work. 

The  steam  engine.  —  Steam  is  generated  in  the  boiler  B, 
Fig.  100,  by  the.  heat  of  the  fire  under  the  boiler.  It  passes 


FIG.  100.  —  The  boiler  and  steam  engine. 

from  the  boiler  through  the  steam  pipe  P,  and  into  the  steam 
chest  S,  of  the  engine.  From  here  it  passes  first  into  one  end 
of  the  cylinder  and  then  into  the  other.  It  is  in  the  cylinder  C 
that  heat  is  turned  into  work.  The  live  steam  from  the  boiler 


SOURCES  OF  HEAT.      HEAT  AND  WORK 


155 


forces  the  piston  first  in  one  direction  and  then  in  the  other, 
and  in  doing  so  forces  the  shaft,  pulley,  and  flywheel  to  re- 
volve. The  machinery  to  be  operated  is  driven  by  a  belt  on 
the  pulley  or  flywheel.  Thus  the  steam  does  work  in  turning 
machinery. 

When  live  steam  from  the  boiler  forces  the  piston  in  one 
direction  or  the  other,  it  does  work,  and  part  of  its  heat  is  turned 
into  this  work.  This  explains 
how  the  steam  engine  turns 
heat  into  work. 

Let  us  now  study  the  steam 
chest  more  closely  and  learn 
how  steam  is  admitted  to  the 
cylinder,  first  at  one  end  and 
then  at  the  other,  and  how 
the  exhaust  steam  is  allowed 
to  escape.  This  is  controlled 
by  a  slide  valve  V.  This 
valve  moves  in  the  steam 
chest  and  is  so  connected  to 
the  shaft  that  it  is  always  be- 
tween one  fourth  and  one  half  stroke  ahead  of  the  piston. 
When  the  slide  valve  V  is  in  the  position  shown  in  Fig.  101, 
live  steam  enters  the  cylinder  by  the  port  p  on  the  left.  The 
exhaust  steam  escapes  by  the  port  p'  on  the  right  and  leaves 
the  engine  through  the  exhaust  pipe  e.  When  the  piston 
moves  to  the  right,  the  slide  valve  moves  to  the  left,  the  live 
steam  is  then  admitted  to  the  cylinder  by  the  right  port  and 
the  exhaust  steam  escapes  by  the  left  port.  The  exhaust  steam 
is  cooler  than  the  live  steam  because  part  of  the  heat  of  the 
live  steam  has  been  turned  into  the  work  done  in  driving  the 
machinery. 

The  gas  engine  and  gasoline  engine.  —  Gas  engines  and  gaso- 
line engines  are  the  same  in  principle.  In  each,  energy  is  de- 
rived from  the  explosion  of  a  mixture  of  gas  and  air.  We  shall 


FIG.   101.  — The  cylinder  and  steam  chest 
of  a  steam  engine. 


156 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.   102.  —  Gasoline 
engine  at  work. 


study  the  gasoline  engines  because  they  are 
the  more  common.  They  are  used  to 
drive  motor  cars,  motor  boats,  aeroplanes, 
small  pumping  plants,  etc.  A  vertical  air- 
cooled  gasoline  engine  is  shown  in  Fig.  102. 
It  is  connected  with  a  pump  jack  and 
pump. 

The  common  type  of  a  gasoline  engine  is 
the  four-cycle  engine.  It  is  called  this 
because  it  makes  four  strokes  (two  revolu- 
tions) for  each  power  stroke.  These  four 
strokes  are  represented  in  Fig.  103.  They 
are  caHed  the  charging  stroke,  the  com- 
pression stroke,  the  power  stroke,  and  the 
exhaust  stroke.  Since  there  is  only  one 
power  stroke  in  four  strokes,  a  gasoline 
engine  must  be  equipped  with  one  or  two 
heavy  flywheels.  It  is  the  momentum  of 

these  wheels  which  drives  the  engine  between  power  strokes. 
The  charging  stroke  is  represented  in  (i).     The  flywheel  is 

pulling  the  piston  down.     This  leaves  a  vacuum  in  the  cylinder, 

and  the  pressure  of  the  atmosphere  forces  open  the  intake  valve 

7  and  forces  air  and  gasoline  vapor  into  the  cylinder  from  the 

carburetor    (not 

shown).         This 

stroke  charges  the 

cylinder     with     a 

mixture  of  gasoline 

vapor  and  air. 
On    the     next 

stroke  (2)  the  fly- 
wheel    forces    the 

piston    up     again. 

The    intake    valve 

closes,  and  the  mix- 


I    E 


CHARGING 
STROKE 

CO 


COMPRESSION 
STROKE 


(2) 


FIG.  103.  —  The  four  strokes  of  a  four-cycle 
gasoline  engine. 


SOURCES  OF   HEAT.      HEAT  AND  WORK  157 

ture  is  compressed  to  about  one  fifth  of  its  volume.  This  is 
the  compression  stroke. 

When  the  piston  on  the  compression  stroke  is  at  or  near  its 
highest  point,  an  electric  spark  is  produced  in  the  compressed 
mixture  (by  a  device  not  shown).  This  ignites  the  mixture, 
and  the  force  of  the  explosion  drives  the  piston  down.  This  is 
the  power  stroke  (3). 

On  the  next  stroke  the  flywheel  forces  the  piston  up  and  the 
burned  gases  are  forced  out  through  the  exhaust  valve  E,  which 
is  opened  automatically  during  the  stroke.  This  is  the  exhaust 
stroke  (4). 

On  the  next  sfroke  the  cylinder  is  charged  again,  and  these 
operations  are  repeated  as  long  as  the  engine  runs. 

Heat  turned  to  work.  —  When  the  mixture  of  gasoline  vapor 
and  air  is  burned  in  the  cylinder,  a  large  amount  of  heat  is  pro- 
duced, and  the  mixture  is  raised  to  a  very  high  temperature. 
As  the  mixture  drives  the  piston  down,  it  does  work  and  part 
of  its  heat  is  turned  into  this  work.  The  mixture  is  cooler  at 
the  end  of  the  power  stroke  than  at  the  beginning  because  part 
of  its  heat  has  been  turned  into  work. 

Horse  power.  —  The  power  of  an  engine  is  the  amount  of 
work  it  can  do  in  a  given  time.  The  common  unit  of  power  is 
the  horse  power.  An  engine  is  working  at  the  rate  of  i  h.  p.  when 
it  does  33,000  foot  pounds  of  work  per  minute. 

The  horse  power  of  an  engine  is  calculated  by  means  of  the 

fonnula:  PXLXAXN 

Horse  power  =  F*L*A  *  N 
33000 

where  P  is  the  mean  effective  pressure  in  pounds  per  square 
inch  in  the  cylinder,  L  is  the  length  of  the  stroke  in  feet,  A  is  the 
area  of  the  piston  in  square  inches,  and  N  is  the  number  of  power 
strokes  per  minute. 

Example.  A  steam  engine  has  a  mean  effective  pressure 
of  30  Ib.  per  square  inch.  The  length  of  stroke  is  i  ft.  The 
area  of  the  piston  is  55  sq.  in.  and  the  engine  makes  240 


158  PHYSICS  OF  THE  HOUSEHOLD 

power  strokes  per  minute.     What  is  the  horse  power  of  the 
-gine?  30  X  i  X  SS  X  ^  „„. 

33000 

Heat  and  work.  —  By  the  work  of  Dr.  Joule  of  Manchester, 
between  1843  and  1849,  and  of  Professor  Rowland  of  Baltimore 
in  1879  it  has  been  learned  that  the  relationship  between  heat 
and  work  is  as  follows:  When  i  B.  T.  U.  of  heat  is  turned  into 
work,  it  produces  778  foot  pounds  of  work.  That  is,  the  small 
amount  of  heat  required  to  warm  i  Ib.  of  water  i°  F.  produces 
778  foot  pounds  of  work.  When  i  calorie  of  heat  is  turned 
into  work,  it  produces  42,700  gram  centimeters  of  work.  The 
work  done  when  one  heat  unit  is  turned  into  work  is  known 
as  the  Mechanical  Equivalent  of  Heat. 

Cost  of  work.  —  The  best  steam  engines  use  about  i  Ib.  of  coal 
per  horse  power  per  hour.  With  coal  at  $4  a  ton,  the  cost  of 
i  h.  p.  hour  of  work  is  .2  ct.  The  ordinary  steam  engine  uses 
about  5  Ib.  of  coal  per  horse  power  per  hour.  The  cost  of 
i  h.  p.  hour  of  work  then  is  about  i  ct. 

A  gasoline  engine  uses  about  yV  gal.  of  gasoline  per  horse 
power  per  hour.  With  gasoline  at  20  ct.  a  gallon,  i  h.  p.  hour 
of  work  costs  2  ct. 

A  team  of  horses  with  driver  costs  $4  per  day.  If  each  horse 
is  working  at  the  rate  of  i  h.  p.,  the  team  in  10  hr.  does  20 
h.  p.  hours  of  work,  and  the  cost  of  i  h.  p.  hour  is  20  ct. 

A  man,  working  hard,  works  at  the  rate  of  about  -^  h.  p.  and 
in  a  day  of  10  hr.  he  does  yV  X  10  =  i  h.  p.  hour  of  work.  If  his 
wage  is  $1.50  per  day,  the  cost  of  i  h.  p  hour  of  work  done  by  a 
man  is  $1.50. 

Cost  of  i  h.  p.  Hour  of  Work 

By  best  steam  engine .2  ct. 

By  ordinary  steam  engine i  ct. 

By  gasoline  engine 2  ct. 

By  horse 20  ct. 

By  man 150  ct. 


SOURCES  OF  HEAT.      HEAT  AND  WORK  159 

Heat  engines  are  of  great  service  to  the  human  race,  not 
only  because  they  free  the  race  from  heavy  drudgery,  but  also 
because,  as  we  see  from  the  above  table,  they  do  work  at  a 
much  smaller  cost  than  it  can  be  done  by  human  beings. 

EXERCISES 

1.  If  2  scuttles  of  coal  per  day  are  used  in  the  kitchen  range,  and 
the  coal  in  each  scuttle  weighs  12^  lb.,  how  long  will  a  ton  of  coal  (2000 
lb.)last? 

2.  If  the  coal  (i)  costs  $6  a  ton,  what  is  the  cost  of  fuel  per  day? 

3.  If  hard  wood  costs  $6  a  cord,  how  much  more  expensive  is  it  as 
a  source  of  heat  than  bituminous  coal  at  $6  a  ton  ? 

4.  If   anthracite   coal  costs   $5  a  ton,  how  many  B.T.  U.  of  heat 
are  purchased  for  $i  ? 

5.  If  natural  gas  costs  80  ct.  per  1000  cu.  ft.,  how  many  B.T.  U. 
of  heat  are  purchased  for  $i  ? 

6.  If  electricity  costs  10  ct.  per  kilowatt  hour,  how  many  B.  T.  U. 
of  heat  are  purchased  for  $i  ? 

7.  If  coal  gas  costs  $i  per  1000  cu.  ft.,  how  many  B.T.  U.  of  heat 
are  purchased  for  $i  ? 

8.  If    gasoline  costs    20  ct.  a  gallon,  how  many  B.T.  U.  of  heat 
are  purchased  for  $i  ? 

9.  If  the  boiler  of  a  hot-water  heating  system  absorbs  only  60  per 
cent  of  the  heat  produced  by  the  coal  burned  under  it,  how  much  money 
is  wasted  if  20  T.  of  coal  at  $5  a  ton  are  burned  during  the  winter  ? 

10.  If  bituminous  coal  costs  $5  a  ton  and  electricity  5  ct.  per  kilo- 
watt  hour,  calculate  the  number  of  B.  T.  U.  of   heat  purchased    for 
Si  in  each  case. 

11.  Make  a  diagram  of  a  boiler  and  engine. 

12.  Describe  the  path  of  the  steam  in  the  boiler  and  steam  engine. 
Use  the  diagram  made  in  (n). 

13.  Explain  the  part  played  by  the  slide  valve. 

14.  Make  a  diagram  illustrating  the  four  strokes  of  a  four-cycle  gaso- 
line engine.     Describe  each  stroke. 

15.  Where  is  heat  turned  into  work  in  the  steam  engine? 

16.  Where  is  heat  turned  into  work  in  the  gasoline  engine? 

17.  A  steam  engine  is  working  at  a  mean  effective  pressure  of  36  lb. 
per  square  inch ;    the  length  of  stroke  is  i  ft. ;    the  area  of  the  piston 
is  44  sq.  in.  and  the  engine  makes  250  strokes  per  minute.     At  what 
horse  power  is  the  engine  working  ? 


160  PHYSICS   OF  THE  HOUSEHOLD 

18.  The  water   for  a   country  house  is  pumped  by  a  i  h.  p.  gaso- 
line engine  working  i  hr.  per  day.     What  is  the  cost  of  the  pumping  per 
day  if  gasoline  costs  25  ct.  a  gallon? 

19.  How  long  would  it  take  a  man  to  do  the  same  amount  of  work 
as  in  (18)  if  he  works  at  the  rate  of  .1  h.  p.  ?     What  would  be  the  cost  of 
the  pumping  per  day  if  his  wages  are  $1.50  per  day? 


CHAPTER  XIV 
ELECTRICITY  IN   THE   HOME 

Household  electrical  appliances.  —  In  this  section  we  shall 
study  household  electrical  appliances.  There  are  many  such 
appliances ;  for  example,  the  electric  bell,  electric  iron,  electric 
oven,  electric  stove,  electric  light,  electric  motor,  motor-driven 
sewing  machine,  motor-driven  vacuum  cleaner,  motor-driven 
washing  machine,  and  the  telephone. 

In  the  case  of  each  appliance  we  shall  try  to  learn  "  how  the 
current  works,"  that  is,  how  it  rings  the  bell ;  how  it  heats  the 
iron,  oven,  and  stove;  how  it  turns  the  motor  and  how  it 
operates  the  telephone.  In  other  words,  we  shall  try  to  learn 
what  is  going  on  inside  of  each  device.  In  order  to  do  this 
we  shall  take  up  the  study  of  electricity  in  a  systematic 
manner,  and  when  we  come  to  those  portions  of  the  subject 
which  have  a  bearing  on  one  or  more  of  the  electrical  appli- 
ances used  in  the  home,  we  shall  make  a  study  of  these 
appliances. 

What  is  electricity? — A  natural  question  to  ask  at  the  begin- 
ning of  the  study  of  electricity  is  "  What  is  electricity?  "  and 
the  very  simple  answer  is  "  We  do  not  know."  There  have  been 
many  theories  advanced  regarding  the  nature  of  electricity, 
but  the  fact  remains  that  up  to  the  present  time  we  do  not  know 
what  electricity  is.  We  do  know,  however,  a  great  many  facts 
about  electricity ;  how  it  can  be  produced ;  how  it  acts  under 
different  circumstances ;  and  the  laws  which  describe  its  action. 
It  is  by  means  of  this  knowledge  that  electricity  has  been  made 
a  servant  of  mankind. 

M  161 


1 62  PHYSICS  OF  THE  HOUSEHOLD 

How  electricity  is  produced.  —  The  three  most  important 
methods  of  producing  electricity  are  (i)  by  friction,  (2)  by 
chemical  action  in  an  electric  cell,  (3)  by  means  of  a  dynamo. 

Electricity  produced  by  friction.  —  Electricity  may  be  pro- 
duced by  rubbing  together  any  two  different  substances.  Prob- 
ably you  have  all  produced  electricity  in  this  way  at  one  time  or 
another.  For  example,  when  the  air  is  very  dry,  as  it  often  is 
in  winter,  you  have  noticed  your  hair  trying  to  follow  your  comb, 
also  you  have  noticed  the  crackling  sound  produced  when  you 
stroked  a  cat's  fur,  when  you  separated  a  blanket  from  a  sheet, 
or  when  you  touched  anything  after  having  scraped  your  shoes 
on  the  carpet.  All  of  these  effects  are  produced  by  electricity. 
When  any  two  different  substances  are  rubbed  together,  one 
becomes  charged  with  what  is  known  as  positive  electricity, 
and  the  other  with  negative  electricity.  These  two  kinds  of 
electricity  attract  each  other.  If  the  objects  charged  with 
them  are  brought  near  together,  the  electricities  unite  across 
the  intervening  air  space,  and  give  rise  to  small  electric  sparks 
which  cause  the  crackling  sound. 

The  quantity  of  electricity  produced  by  friction  at  any  one 
time  is  very  small.  It  is  not  large  enough  to  be  of  any  practical 
use  in  the  home,  and  for  this  reason  we  shall  not  study  f rictional 
electricity  further. 

The  electric  current  used  in  the  home  or  elsewhere  is  pro- 
duced either  in  an  electric  cell,  or  by  means  of  a  dynamo.  We 
shall  first  study  the  electric  cell  and  later  on  the  dynamo. 

ELECTRIC  CELLS 

It  is  found  by  experiment  that  an  electric  current  is  produced 
whenever  two  different  metals  are  placed  in  water  or  in  a 
solution  of  an  acid,  a  base,  or  a  salt,  and  then  joined  by  a 
wire. 

Such  an  arrangement  is  known  as  a  simple  electric  cell.  The 
electric  cells  in  actual  use  are  modifications  of  this  simple  cell. 
You  have  probably  heard  electric  cells  called  electric  batteries. 


ELECTRICITY  IN  THE  HOME  163 

The  name  "  battery  "  properly  belongs  to  a  combination  of 
two  or  more  cells,  but  it  is  now  very  commonly  applied  to  a 
single  cell. 

Electric  cells  are  used  in  a  number  of  ways  about  the  home ; 
for  example,  to  ring  electric  bells  and  to  operate  the  telephone. 
The  electric  cell  is  then  the  first  household  electrical  appliance  for 
us  to  study.  Let  us  see  what  takes  place  inside  an  electric  cell. 

The  simple  cell.  —  If  a  plate  of  copper  and  a  plate  of  zinc 
are  placed  in  a  dilute  solution  of  sulphuric  acid,  it  is  found  that 
the  copper  plate  becomes  charged  with  positive  electricity,  and 
the  zinc  plate  with  negative  electricity, 
as  illustrated  in  Fig.  104.  When  the  two 
plates  are  joined  by  a  wire,  the  two 
kinds  of  electricity  flow  along  the  wire 
and  unite.  As  soon  as  the  electricities 
begin  to  leave  the  plates,  more  is  formed 
by  the  chemical  action  in  the  cell,  and 
thus  there  is  a  continuous  flow  of  elec- 
tricity through  the  wire.  It  is  assumed  ,., 

'  FIG.  104.  —  A  simple  cell. 

that  the  current  is  flowing  in  the  direc- 
tion the  positive  electricity  moves,  that  is,  from  the  copper 
to  the  zinc  through  the  wire  and  from  the  zinc  to  the  copper 
through  the  liquid. 

There  have  been  many  experiments  made  on  electric  cells, 
and  as  a  result  the  following  facts  about  them  have  been  learned. 

In  every  electric  cell,  (i)  there  must  be  two  different  metals; 

(2)  the  liquid  must  dissolve  one  more  readily  than  the  other; 

(3)  the  more  soluble  metal  is  always  charged  with  negative 
electricity  and  the  other  with  positive  electricity. 

Chemical  action  in  a  simple  cell.  —  The  chemical  action  in 
the  simple  cell  is  as  follows.  The  zinc  dissolves  in  the  sulphuric 
acid  and  forms  zinc  sulphate  and  hydrogen.  The  chemical 
equation  is 

Zn  +  H2S04  =  ZnSO4  +  H2 
Zinc  -f-  sulphuric  acid  =  zinc  sulphate  +  hydrogen 


164 


PHYSICS   OF  THE  HOUSEHOLD 


The  current  given  by  the  cell  stops  when  all  the  zinc  is  dis- 
solved, or  when  all  the  sulphuric  acid  is  turned  to  zinc  sulphate. 
We  should  naturally  expect  that  the  hydrogen  gas  formed  by 
the  action  of  the  sulphuric  acid  on  the  zinc  plate  would  appear 
at  the  zinc  plate,  but  this  is  not  the  case.  When  the  current 
is  running,  the  hydrogen  travels  with  (or  carries)  the  current 
through  the  liquid  and  is  deposited  on  the  copper  plate.  The 
copper  plate  is  not  dissolved  by  the  liquid. 

Polarization.  —  If  the  current  is  allowed  to  run  for  some  time 
in  a  simple  cell,  it  is  found  that  the  current  gradually  decreases 
in  strength,  and  at  the  same  time  it  is  noticed  that  a  great  deal 
of  hydrogen  gas  collects  on  the  copper  plate.  A  gas  is  a  very 
poor  conductor  of  electricity,  and,  therefore,  the  layer  of  hydro- 
gen on  the  copper  plate  gradually  stops  the  current.  When 
the -current  in  a  cell  stops  from  this  cause,  the  cell  is  said  to  be 
polarized. 

Polarization  is  a  serious  defect  in  an  electric  cell,  and  many 
methods  of  overcoming  it  have  been  invented.  We  may 
now  study  a  few  of  the  more  important  cells,  and  learn  how 
polarization  has  been  prevented  or  decreased  in  each. 

The  Daniell  cell.  —  In  the  Daniell 
cell  (Fig.  105)  polarization  is  pre- 
vented almost  entirely.  The  metals 
are  zinc  and  copper.  There  are  two 
liquids,  a  dilute  solution  of  sulphuric 
acid  or  of  zinc  sulphate,  and  a  strong 
solution  of  copper  sulphate  (blue 
vitriol) .  The  zinc  plate  is  placed  in 
a  porous  earthenware  cup  which 
holds  the  dilute  solution  of  sulphuric 
acid  or  of  zinc  sulphate.  The  cop- 
per plate  surrounds  the  porous  cup 
and  is  immersed  in  the  strong  solu- 
tion of  copper  sulphate. 
FIG.  105.— The  Daniell  cell.  The  chemical  action  occurs  in  two 


ELECTRICITY   IX  THE  HOME 


parts,  as  follows.  The  zinc  dissolves  in  the  sulphuric  acid,  and 
forms  zinc  sulphate  and  hydrogen,  as  in  the  simple  cell.  The 
hydrogen  moves  towards  the  copper  plate  and  when  it  passes 
through  the  porous  cup,  comes  in  contact  with  the  copper  sul- 
phate. It  acts  on  the  copper  sulphate  and  forms  sulphuric  acid 
and  copper. 

H2  +  CuSO4  =  H2SO4  +  Cu 
Hydrogen  -f-  copper  sulphate  =  sulphuric  acid  +  copper 

Thus  copper  is  deposited  on  the  copper  plate  instead  of  hy- 
drogen, and  since  copper  is  an  excellent  conductor  of  electricity, 
there  is  practically  no  polarization. 

The  gravity  cell.  —  The  gravity  cell  (Fig.  106)  is  a  modifica- 
tion of  the  Daniell  cell.     The  copper  plate  is  placed  on  the 
bottom  of  the  cell  and  is  covered 
with  crystals  of   copper   sulphate 
and    with    a    strong    solution    of 
copper    sulphate.      This    solution 
fills  the  lower  half  of  the  cell. 

The  zinc  plate  is  suspended  from 
the  side  of  the  cell  near  the  top 
and  is  shaped  like  a  crow's  foot ; 
hence  the  cell  is  sometimes  called 
a  "  crowfoot "  cell.  The  zinc  is  im- 
mersed in  a  dilute  solution  of  sul- 
phuric acid  or  of  zinc  sulphate. 
The  chemical  action  is  the  same 
as  that  in  the  Daniell  cell. 

The  copper  sulphate  solution  is  heavier  than  the  other  solu- 
tion and  thus  the  liquids  are  kept  apart  by  the  force  of  gravity 
—  hence  the  name  "  gravity  cell." 

The  Daniell  cell  and  the  gravity  cell  are  important  cells  be- 
cause they  can  be  used  on  "  closed  circuits,"  that  is,  on  cir- 
cuits which  use  a  steady  current  for  a  long  time. 

The  Leclanche  cell.  —  The  Leclanche  cell  (Fig.  107),  is  in  very 
common  use  in  houses,  especially  for  operating  doorbells,  etc. 


FIG.   106.  —  The  gravity  cell. 


£66 


PHYSICS   OF  THE  HOUSEHOLD 


The  "  metals  "  are  zinc  and  carbon  and  the  liquid  a  strong  soh> 

tion  of  ammonium  chloride  ("  sal  ammoniac  ")•     Polarization 

is  decreased  as  follows. 
The  carbon  plate  is  placed 
in  a  porous  cup  and  is 
surrounded  by  manganese 
dioxide  and  particles  of 
carbon.  The  manganese 
dioxide  decreases  polariza- 
tion by  oxidizing  the  hy- 
drogen to  form  water. 
The  particles  of  carbon 
are  distributed  through 
the  manganese  dioxide 
and  may  be  considered  as 
part  of  the  carbon  plate. 

The  Leclanche  cell  gives 
a  strong  current  for  a 
short  time ;  but  since  the 

manganese  dioxide  is  not  rapid   in  its  action,  the  hydrogen 

accumulates  and  decreases  the  current.     If  the  cell  is  left  on 

open  circuit  for  a  short  time,  the  manganese  dioxide  oxidizes  the 

hydrogen  and  the  cell  is  again  ready  for  use. 
The  cell  is  used  only  on  open-circuit  work, 

that  is,  on  work  where  the  current  is  needed 

only  for  a  short  time  and  the  cell  has  time 

to   recover   before   it   is   needed  again,  for 

example,  on  electric  bells  and  the  telephone. 
The  dry  cell.  —  The  most  convenient  cell 

for  use  in  the  home  is  the  dry  cell  (Fig.  108). 

It  is  portable;    the  liquids  cannot  spill  or 

evaporate ;  it  gives  a  large  current  and  has 

a  useful  life  of  one  or  two  years. 

There  are  many  forms  of  the  dry  cell  on 

the  market,  but  they  are  all  modifications  of 


FIG.  107.  —  The  Leclanche  cell. 


FIG.  108.  — The  dry 
cell. 


ELECTRICITY  IN  THE   HOME 


167 


the  Leclanche  cell.  The  metals  are  zinc  and  carbon  and  the 
chief  constituent  of  the  liquid  is  ammonium  chloride.  The 
arrangement  is  as  follows.  The  zinc  plate  is  the  whole  outer 
cylinder  Z  which  holds  the  cell ;  next  to  this  is  a  strong  solution 
of  "  sal  ammoniac  "  S  mixed  with  plaster  of  Paris  to  make  a 
stiff  paste.  Inside  of  this  is  a  cylinder  of  blotting  paper  B 
which  takes  the  place  of  the  porous  cup  used  in  the  Leclanche 
cell.  The  blotting  paper  cylinder  holds  the  carbon  plate  C  and 
the  manganese  dioxide  and  carbon  particles  which  surround 
the  carbon.  The  whole  cell  is  moistened  and  the  top  is  sealed 
with  pitch  P.  The  cell  is  a  moist  cell  rather  than  a  dry  cell. 

Electromotive  force,  resistance,  and  current.  —  The  force 
which  moves  electricity  through  a  cell  and  the  outer  circuit 
is  known  as  electromotive  force.  It  is  measured  in  wits.  The 
electromotive  force  of  a  cell  depends  on  the  nature  of  the  metals 
and  on  the  nature  of  the  liquid. 

When  electricity  passes  through  any  substance,  solid  or 
liquid,  it  encounters  a  certain  amount  of  resistance,  which  is 
measured  in  ohms.  The  resistance  of  a  cell  depends  on  the 
nature  of  the  plates  and  of  the  liquid,  also  on  the  size  of  the 
plates  and  the  distance  between  them. 

The  current  produced  by  a  cell  is  measured  in  amperes.  It 
is  found  by  dividing  the  electromotive  force  in  volts  by  the  total 
resistance  in  ohms. 

The  terms  volt,  ohm,  and  ampere  are  explained  further  in 
the  chapter  on  electrical  terms  (Chapter  XEX). 

We  give  below  the  approximate  volts,  ohms,  and  amperes  of 
the  cells  mentioned  above. 


ELECTRIC  CELLS 

DANIELL 

GRAVITY 

LECLANCHE 

DRY 

Electromotive  force  in  volts  . 

I 

I 

i-5 

1-5 

Approximate  resistance  in  ohms 

I 

i 

\ 

A 

Current  in  amperes  on  short  cir- 

cuit      

I 

2 

2 

15 

1 68  PHYSICS  OF  THE  HOUSEHOLD 

Conductors  and  insulators.  —  Substances  through  which 
electricity  passes  readily  are  called  conductors  of  electricity. 
Substances  through  which  electricity  does  not  pass  readily  are 
called  nonconductors  or  insulators. 

Metals,  carbon,  and  solutions  of  acids,  bases,  and  salts  are 
conductors.  The  nonmetals  are  insulators.  There  is  no  hard 
and  fast  line  between  conductors  and  insulators.  All  conduc- 
tors offer  some  resistance  to  the  electric  current,  and  all  insula- 
tors conduct  electricity  to  some  extent.  The  metals  are  the  best 
conductors,  and  of  the  common  metals  silver  is  the  best.  They 
stand  in  the  following  order:  silver,  copper,  gold,  aluminium, 
zinc,  iron,  platinum,  nickel,  tin,  lead,  mercury.  The  common 
insulators  are  sulphur,  glass,  porcelain,  hard  rubber,  mica, 
shellac,  wood,  silk,  oils,  cotton,  and  air. 

All  electrical  appliances  make  use  of  both  conductors  and 
insulators.  For  example,  in  the  electric  cells  which  we  have  been 
studying,  the  metal  plates  and  solutions  are  conductors.  The 
glass  jars  holding  the  solutions  are  insulators.  The  wire  with 
which  we  joined  the  plates  of  the  cells  is  made  of  the  conductor, 
copper ;  the  covering  of  the  wire  is  the  insulator,  cotton  or  silk. 
Telephone  wires  are  made  of  the  conductor,  copper  ;  telegraph 
wires  of  the  conductor,  iron.  They  are  attached  to  the  poles  by 
means  of  insulators  made  of  glass  or  porcelain. 

As  we  proceed  with  the  study  of  electricity  we  shall  find 
that  in  every  case  both  conductors  and  insulators  are  used  in 
the  construction  of  electrical  appliances. 

EXERCISES 

1.  Describe  the  simple  electric  cell. 

2.  Describe  the  Daniell  cell.     Make  a  drawing. 

3.  Describe  the  gravity  cell.     Make  a  drawing. 

4.  Describe  the  Leclanche  cell.     Make  a  drawing. 

5.  Describe  the  dry  cell.     Make  a  drawing. 

6.  Name  four  conductors  and  four  insulators. 


CHAPTER  XV 


MAGNETISM   AND   THE   ELECTROMAGNET 

BEFORE  we  proceed  with  our  study  of  electrical  appliances, 
it  will  be  necessary  to  learn  something  about  magnets,  because 
magnets  are  used  in  many  of  these  appliances. 

Natural  and  artificial  magnets.  —  In  many  parts  of  the  earth 
an  iron  ore  is  found  which  has  the  property  of  attracting  parti- 
cles of  iron  or  steel.  A  piece  of  this  ore  is  known  as  a  natural 
magnet  or  lodestone.  If  a  piece  of  steel  is  stroked  with  a  natu- 
ral magnet,  it  acquires  the  property  of  attracting  iron  or  steel, 
and  is  known  as  an  artificial  magnet.  Other  ways  of  making 
artificial  magnets  are  described  below. 

Magnetic  substances.  -^Jron  and  steel  a.™  fV  only  sub- 
stances which  have  strong  magnetic  properties.  Cobalt,  nickel, 
and  certain  alloys  are  slightly  magnetic ;  other  substances  are 
practically  nonmagnetic. 

Magnet  poles.  —  If  we  dip  a  magnet  into  iron  filings,  we  find 
that  the  filings  cling  to  the  ends,  but  scarcely  at  all  to  the 
middle  of  the  magnet 
(Fig.  109).  The 
places  near  the  end 
where  the  magnetic 
effect  is  strong  are 
called  the  poles  of  the 
magnet. 

If  a  magnet  is  sus- 
pended in  a  stirrup  as 
in  Fig.  ^i  10,  it  comes  FlG  IOQ.  _The  magnetic  effett  of  a  magnet  1S 

to  rest  in  a  north  and  stronger  at  the  ends  than  at  the  middle. 

169 


1 70  PHYSICS  OF  THE  HOUSEHOLD 

south  position.     The  end  which  points  to  the  north  is  called 

the  north-seeking  pole  or  simply  the  north  pole  of  the  magnet. 

The  other  end  is  called  the  south  pole. 

If  we  suspend  a  magnet  in  a  stirrup  and  then  bring  the  north 

pole  of  another  magnet  near  the  north  pole  of  the  suspended 
magnet,  we  find  that  they  repel  each 
other.  If,  however,  we  bring  a  south 
pole  to  the  north  pole  of  the  suspended 
magnet,  we  find  that  they  attract  each 
other.  Similarly  it  is  found  that  two 
south  poles  repel  each  other.  From 
experiments  of  this  kind  we  have  the 

FIG.    no. —  A   suspended    rule:     Like   magnet   p0les   repel   each   other 
magnet  points  north  and 

south.  and    unlike    magnet    poles    attract    each 

other. 

Magnetic  induction.  —  //  an  iron  object  is  brought  near  or 
into  contact  with  a  magnet,  it  becomes  a  magnet  (Fig.  111). 
This  action  of  a  magnet  on  iron  or  steel  is  known  as  magnetic 
induction. 

If  the  object  is  made  of  soft  iron,1  it  is  readily  magnetized,  but 
loses  all  of  its  magnetism  as  soon  as  the  magnet  is  removed. 
If  it  is  made  of  steel,  it  is  not  readily  magnetized,  but  retains  some 
of  its  magnetism  when  the  magnet  N 
is  removed.  A  substance  which 
becomes  strongly  magnetic  when 
near  a  magnet  is  said  to  possess 
permeability;  a  substance  which  resists  magnetization  or  de- 
magnetization is  said  to  possess  retentivity.  Soft  iron  is  more 
permeable  than  steel,  but  has  less  retentivity. 

Magnetic  lines  of  force.  —  It  is  important  that  we  learn  the 
meaning  of  the  term  magnetic  line  of  force  because  we  use 
it  to  explain  the  working  of  many  electrical  appliances. 

A  magnetic  line  of  force  is  defined  as  the  line  along  which  a 
free  north  pole  would  move  in  going  from  the  north  pole  to  the 

' l  Soft  iron  is  iron  practically  free  from  cafbon. . 


MAGNETISM   AND   THE   ELECTROMAGNET 


171 


south  pole  of  a  magnet.  We  cannot  obtain  a  free  north  pole, 
that  is,  a  north  pole  separate  from  a  south  pole,  but  we  can  use 
a  small  magnetic  needle  to 
mark  out  the  magnetic  lines 
of  force  of  a  magnet. 

If  a  small  compass  is  placed 
near  the  north  pole  of  a 
magnet  and  then  moved  a 
short  distance  at  a  time  in 
the  direction  its  north  pole 
points,  it  travels  from  the 
north  to  the  south  pole  along  FIG  II2  _The  compass  moves  along  a 

a     magnetic     line     of     force,  magnetic  line  of  force. 

Fig.  112. 

Magnetic  field  of  a  magnet.  —  The  region  about  a  magnet  in 
which  its  effect  can  be  detected  is  called  the  magnetic  field  of 
the  magnet.  The  magnetic  field  is  filled  with  lines  of  force. 
The  arrangement  of  these  lines  can  be  determined  as  follows: 
Place  a  magnet  under  a  pane  of  glass  and  sift  iron  filings  over 
the  glass.  Tap  the  glass  gently  and  the  filings  arrange  them- 
selves along  the  lines  of 
force.  The  lines  of  force 
about  a  bar  magnet  are 
shown  in  Fig.  113  and  about 
a  horseshoe  magnet  in  Fig. 
114. 

Each  iron  filing  becomes 
a  magnet  by  induction  when 
it  enters  the  field  of  the 
magnet.  It  is  in  reality  a 
small  compass;  and  when 
the  glass  is  tapped,  the  riling  is  turned  by  the  magnet  until 
it  is  in  the  direction  of  the  magnetic  line  of  force  at  the 
point  it  occupies.  It  will  be  noticed  that  the  direction  of  a 
magnetic  line  of  force  at  any  point  is  the  direction  of  the  force 


FIG.  113.  —  The  magnetic  field  about  a 
bar  magnet. 


172 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  114. 


—  The  magnetic  field  about  a 
horseshoe  magnet. 


B 


of  the  magnet  at  this 
point.  A  magnetic  line  is 
considered  to  move  from 
north  to  south  outside  of 
the  magnet  and  from 
south  to  north  in  the 
magnet. 

Strength  of  a  magnet 
pole.  —  A  magnet  pole  is 
of  unit  strength  when  it 
repels  an  exactly  similar 
pole  at  a  distance  of  i  cm. 

with  a  force  of  i  dyne.  (See  Chapter  XXX  for  definition  of  dyne.) 
The  magnetic  lines  of  force  are  used  to  represent  the  strength 
of  a  magnet.  If  the  magnet  has  unit  strength,  it  is  represented 
as  sending  one  magnetic  line 
of  force  through  each  square 
centimeter  at  a  distance  of 
one  centimeter.  If  it  has 
a  strength  of  100,  it  is  repre- 
sented as  sending  100  lines  of  force  through  each  square 
centimeter  at  a  distance  of  i  cm.,  etc. 

We  see,  then,  that  the 
magnetic  lines  of  force  are 
used  to  represent  the  direc- 
tion and  strength  of  the 
magnetic  field  about  a 
magnet. 

How  artificial  magnets 
are  made.  —  A  piece  of 
steel  can  be  made  a  magnet 
by  stroking  it  in  one  direc- 
tion with  one  pole  of  a 
strong  magnet,  Fig.  115. 
If  AB  is  stroked,  in  the 


FIG.  115.  —  Making  a  magnet. 


FIG.  116.  —  Making  a  magnet  by  means 
of  an  electromagnet. 


MAGNETISM   AND  THE   ELECTROMAGNET  173 

direction  AB  only,  with  a  north  pole,  the  end  B  becomes  a 
south  pole  and  A  a  north  pole. 

If  a  piece  of  steel  is  laid  on  the  poles  of  a  strong  electro- 
magnet, Fig.  116,  and  hammered,  it  becomes 
a  strong  magnet.     The  end  in  contact  with 
the  north  pole  becomes  a  south  pole  and 
vice  versa.  FlG-  1 17.  — Making  a 

e  .  .  ,      .,.    .        ,    ,     ,        magnet  by  means  of 

If  a  piece  of  steel  is  wound  with  insulated      an  eiectric  current. 
wire  (Fig.  117),  and  a  strong  electric  current 
is  passed  through  the  wire,  the  steel  becomes  a  strong  magnet. 

These  are  three  ways  of  making  a  magnet. 

Theory  of  molecular  magnets.  —  To  account  for  certain  facts 
about  magnets  it  is  assumed  that  each  molecule  of  iron  or  steel 
is  a  magnet.  This  is  the  theory  of  molecular  magnets.  When  the 
iron  or  steel  is  a  magnet,  the  small  molecular  magnets  are 
supposed  to  point  all  in  the  same  direction;  Fig.  1 20.  When  the 
iron  or  steel  is  not  a  magnet,  the  small  molecular  magnets  are 
supposed  to  point  in  different  directions.  In  the  latter  case, 
there  would  be  an  equal  number  of  molecular  north  and  south 
poles  pointing  in  any  direction,  and  the  one  would  neutralize 
the  other. 

We  can  represent  one  pole  neutralizing  the  effect  of  an  oppo- 
site pole  by  the  experiment  shown  in  Fig.  118.     A  nail  is  at- 
, ,   tracted  by  a  north  pole  of  one 

|"c  M]  ~ 

IN  si magnet,  but  if  we  slide  a  south 

A  pole   of   another   magnet   up   to 

the    north    pole,    the    nail   falls. 
This  shows  that  the  south  pole 

FIG.  1 1 8.  —  Opposite  poles  neutralize 

one  another.  neutralizes  the  effect  of  the  north 

pole. 

The  theory  of  molecular  magnets  explains  the  following : 
When  a  piece  of  steel  is  stroked  with  the  north  pole  of  a  mag- 
net, the  end  last  touched  is  a  south  pole  because  the  south  poles 
of  the  molecular  magnets  are  left  pointing  in  this  direction. 
When  a  piece  of  steel  is  magnetized  by  an  electromagnet, 


174 


PHYSICS  OF  THE  HOUSEHOLD 


the  poles  of  the  steel  are  opposite  to  those  of  the  electromagnet 
because  the  north  pole  of  the  electromagnet  attracts  the  south 
poles  of  the  molecular  magnets  and  the  south  pole  of  the  electro- 
magnet attracts  the  north  poles  of  the  molecular  magnets. 

A  magnet  shows  no  magnetic  effect  at  the  middle ;  but  if  it  is 
broken  at  this  point,  each  piece  is  a  magnet  and  two  strong  poles 


n  .:.     s,v   S  ~zt 


FIG.  119.  —  Breaking  a  magnet  produces  new  magnetic  poles. 

appear  at  the  point  which  formerly  had  no  magnetic  effect 
(Fig.  119).  When  the  magnet  is  broken,  the  ends  of  the  molec- 
ular magnets  exposed  on  one  piece  are  all  north  poles  and  on 
the  other  all  south  poles,  Fig.  120.  Thus  two  new  poles  are 
formed. 

A  magnet  can  be  demagnetized  by  heating  it  red  hot,  because 
the  molecular  magnets  are  set  in  rapid  vibration  and  assume 
a  heterogeneous  arrangement.  Thus  at  any  point  an  equal 


N 


S'N' 


11           X 

n      s 

n      s 

n       s 

n      s 

n      s 

n      si 

H           .v 

n       s 

n      s 

n     s 

n       s 

n      .<? 

n      s 

n      s 

11           $ 

n       s 

n      s 

n     s 

n      s 

n      s 

n      s 

*      s 

fl 

N 


SN 


FIG.  120.  — The  molecular  poles  exposed  on  one  side  of  the  break 
are  all  south  poles,  on  the  other  side  all  north  poles. 

number  of  molecular  north  and  south  poles  point  in  a  given 
direction  and  they  neutralize  one  another. 

The  mariner's  compass.  —  The  simplest  form  of  mariner's 
compass  (Fig.  121),  is  a  magnetized  needle  fastened  under  a 
circular  card.  The  upper  surface  of  the  card  is  divided  by  radii 
into  thirty-two  divisions.  These  are  called  the  points  of  the 
compass. 


MAGNETISM  AND   THE   ELECTROMAGNET 


175 


FIG.  121.  —  The  mariner' s  compass. 


In  order  that  the  compass 
may  remain  horizontal  in  spite 
of  the  rolling  and  pitching  of 
the  ship,  it  is  supported  on 
gimbals.  That  is,  the  com- 
pass turns  on  pivots  fastened 
to  a  ring,  and  the  ring  turns 
on  pivots  at  right  angles  to 
those  of  the  compass. 

Permanent  magnets  and 
electromagnets.  —  The  steel 

magnets  we  have  been  studying  are  called  permanent  magnets. 
An  electromagnet  consists  of  a  core  of  soft  iron  magnetized  by 
an  electric  current.  It  is  a  magnet  only  when  the  current  is 
flowing.  Of  the  two,  th  electromagnet  is  the  more  important. 

THE  ELECTROMAGNET 

Oersted's  discovery.  —  In  1819  a  Dane  named  Oersted  dis- 
covered that  if  an  electric  current  is  passed  over  or  under  a 
magnetic  needle,  the  needle  tends  to  set  itself  at  right  angles 
to  the  current  (Fig.  122).  This  was  an  important  discovery 


FIG.  122.  —  Oersted's  experiment. 

because  it  showed  for  the  first  time  that  there  is  a  relation  be- 
tween electricity  and  magnetism. 
If  the  needle  is  under  the  wire,  the  north  pole  turns  in  one 


1  76  PHYSICS  OF  THE  HOUSEHOLD 

direction  ;  if  it  is  above  the  wire,  it  turns  in  the  opposite  direction. 
If  the  needle  is  kept  in  one  position  and  the  direction  of  the 
current  is  reversed,  the  direction  in  which  the  north  pole  turns 
is  reversed. 

Magnetic  field  about  a  current.  —  It  is  now  known  that  a 
wire  carrying  a  current  is  surrounded  by  a  magnetic  field,  and 

that  the  needle*  turns  because  it  tends  to  set 

itself  parallel  to  this  field. 

The  magnetic  field  about  a  wire  carrying  a 

current  can  be  shown  by  means  of  the  experi- 

ment illustrated  in  Fig.  123.     A  wire  carrying 

4  a  strong  current  passes  through  a  piece  of  card- 

board, and   iron   filings   are   sprinkled   on   the 
cardboard.     If  the  cardboard  is  tapped  gently, 
Flmagneti7fiekl   the    nlings    arrange    themselves    in    concentric 
about  a  wire    circles  about  the  wire.     These  circles  show  the 
^yingacur~   paths  of  the  magnetic  lines  of  force  about  the 

wire. 

Direction  of  the  lines  of  force.  —  A  magnetic  needle  turns 
until  its  north  pole  points  in  the  direction  of  the  magnetic  lines 
of  force.  A  small  compass  can  be  used  to  find  the  direction  of 
the  magnetic  lines  of  force  about  a  wire  carrying  a  current. 
It  is  found  that  the  direction  of  the  lines 
is  reversed  when  the  current  is  reversed. 
The  following  rule  will  be  found  a  con- 
venient aid  in  remembering  the  direction 
of  the  magnetic  lines  of  force  about  a 
wire:  "  Grasp  the  wire  in  the  right  hand  FlG-  124.—  The  magnetic 

field  about  a  coil  carry- 

with   the  extended   thumb   pointing   in       ing  a  current. 
the  direction  of  the  current;  the  fingers 

then  point  in  the  direction  of  the  magnetic  lines  of  force." 
In  Fig.  124  the  current  passes  through  a  coil  of  wire  in  the 
direction  shown  by  the  arrows.  By  means  of  the  rule  given 
above  we  find  that  the  magnetic  field  about  one  side  of  the  coil 
is  in  the  opposite  direction  of  that  about  the  other.  It  will  be 


^-g 

* 


MAGNETISM   AND  THE  ELECTROMAGNET 


177 


noticed,  however,  that  at  the  middle  of  the  coil  both  fields  move 
in  the  same  direction.  A  magnetic  needle  placed  at  the  middle 
is  turned  in  the  same  direction  by  the  magnetic  field  of  both 
sides  of  the  coil.  In  fact,  the  magnetic  field  about  every  part 
of  the  coil  turns  the  needle  in  the  same  direction  when  it  is  at 
the  middle  of  the  coil. 

The  electromagnet.  —  If  we  wind  a  coil  of  insulated  wire 
around  a  soft  iron  bar  and  pass  a  current  through  the  wire, 


FIG.  125.  —  A  straight  electromagnet. 

the  iron  becomes  a  strong  magnet.  If  we  stop  the  current,  the 
iron  ceases  to  be  a  magnet.  A  soft  iron  bar  wound  with  insu- 
lated wire  is  an  electromagnet. 

The  electromagnet  is  the  most  useful  form  of  magnet  because 
it  is  under  our  control.  It  becomes  a  magnet  when  we  turn  on 
the  current,  and  ceases  to  be  a  magnet  when  we  stop  the  current. 
We  shall  find  that  the  electromagnet  is  an  important  part  of 
all  electrical  appliances  which  are  used  to  produce 
motion. 

If  we  pass  the  current  through  the  coil  in  one 
direction,  the  north  pole  is  at  one  end  of  the 
soft  iron  bar ;  if  we  reverse  the  current,  it  is  at 
the  other  end.  The  position  of  the  north  pole 
depends  on  the  direction  of  the  current  and  on 
the  way  the  wire  is  wound  on  the  soft  iron.  tromagnet. 


17* 


PHYSICS  OF  THE  HOUSEHOLD 


We  shall  find  the  following  rule  an  aid  in  determining  the  posi- 
tion of  the  north  pole  when  we  know  the  direction  of  the  current : 
"  Grasp  the  coil  in  the  right  hand  with  the  fingers  in  the  direc- 
tion the  current  is  flowing,  the  extended  thumb  then  points  to 
the  north  pole  of  the  electromagnet." 

A  straight  electromagnet  is  shown  in  Fig.  125.  If  we  apply 
the  above  rule,  we  find  that  the  north  pole  is  as  shown.  A  horse- 
shoe electromagnet  is  shown  in  Fig.  126. 

APPLICATIONS  or  THE  ELECTROMAGNET 

The  electric  bell.  —  The  electric  bell  is  the  most  common  of 
household  electrical  appliances.  The  diagram,  Fig.  127,  shows 
an  electric  bell  connected  with  a  push  button  and  battery. 

Let  us  see  how  the 
electric  current  rings 
the  bell.  We  shall 
find  that  it  does  so  by 
means  of  an  electro- 
magnet. 

In  the  diagram,  B  is 
an  electric  battery,  P  a 
push  button,  E  an 
electromagnet,  H  the 
bell  hammer,  and  C  the 
point  of  contact  be- 
tween a  set  screw  and 
one  end  of  a  steel 
spring.  The  upper  end 
of  this  spring  holds  a 
soft  iron  bar  a  short  distance  from  the  poles  of  the  electro- 
magnet. 

The  current  rings  the  bell  as  follows :  When  the  button  P  is 
pressed,  the  current  flows  from  the  battery  through  the  elec- 
tromagnet, along  the  steel  spring  and  set  screw,  and  back  to  the 
battery.  When  the  current  starts,  E  becomes  a  magnet  and 


FIG.  127.  —  Electric  bell,  push  button, 
and  battery. 


MAGNETISM    AND   THE   ELECTROMAGNET 


179 


FIG.  128.— The 
push  button. 


draws  over  the  soft  iron  bar.  This  movement  of  the  bar  does  two 
things :  it  makes  H  strike  the  bell  and  it  "'breaks  "  the  current 
at  C.  When  the  current  ceases,  E  is  no  longer  a 
magnet  and  the  spring  draws  the  soft  iron  bar 
back.  This  backward  movement  of  the  bar 
"  makes  "  the  current  again  at  C,  and  the  whole 
operation  is  repeated,  that  is,  the  current  starts, 
E  becomes  a  magnet  and  draws  over  the  bar, 
this  makes  H  strike  the  bell  again  and  also 
"  breaks  "  the  current  at  C  again.  This  opera- 
tion is  repeated  in  rapid  succession  as  long  as 
the  push  button  is  pressed. 

The  push  button.  —  A  diagram  of  the  push 
button  is  shown  in  Fig.  128.  The  wires  are 
attached  to  metal  strips,  one  of  which  is  a 
spring ;  when  the  button  P  is  pushed,  the  spring 
is  brought  into  contact  with  the  lower  metal 
strip  and  the  current  starts. 

Bell  circuits. — The  diagram  (Fig.  129)  shows  the  manner 
in  which  three  bells  and  their  push  buttons  are  joined  to  one 
battery.  The  battery  is  made  up  of  three  cells  joined  in  series. 

^ It  is  usually  placed 

in  the  basement,  and 
the  bells  in  the 
kitchen.  The  but- 
tons, as  shown,  are 
placed  at  the  front 
door,  in  the  dining 
room,  and  in  the 
drawing  room.  If 
we  start  at  the  left 
side  of  the  battery, 
the  current  in  each 
case  runs  as  follows :  from  the  battery  through  the  button  and 
the  bell,  and  back  to  the  battery. 


Drawing  Room    Dining  Room 

FIG.   1 29.  —  Bell  circuits. 


i8o 


PHYSICS  OF  THE  HOUSEHOLD 


The  electric  telegraph.  —  The  tele- 
graph is  one  of  the  most  important 
electrical  servants  of  mankind.  It  is 
another  application  of  the  electro- 
magnet. The  equipment  of  a  tele- 
graph station  usually  consists  of  a 
I  sounder,  key,  relay,  and  batteries. 

The  sounder  (Fig.  130)  gives  the 
message.  The  important  part  of  it 
is  an  electromagnet.  When  the  cur- 
rent enters  the  electromagnet  M ,  the  iron  armature  7  is  drawn 
down.  This  draws  down 
the  bar  B,  and  the  screw 
Cf  strikes  the  brass  piece  D. 
This  makes  a  click.  When 
the  current  stops,  M  ceases 


130. — The  telegraph 
sounder. 


to  be  a  magnet,  and  the 
spring  S  raises  the  arma- 
ture I  and  the  bar  B.  B 


131.  — The  telegraph  key. 


FIG.  132.  —  The  telegraph  relay. 


then   strikes   the   screw  C  and 
makes  another  click. 

The  message  is  sent  by  dots 
and  dashes.  A  dot  is  a  short 
time  between  clicks.  A  dash  is 
double  the  time  of  a  dot  and  a 
double  dash,  treble  the  time. 
The  key  (Fig.  1 3 1 )  is  used  to  send 

the  message,  by  starting  and  stopping  the  current.     The  lever 

L  is  used  to  close  the  key  except  when  a  message  is  being  sent. 
The  relay,  Fig.  132,  is 

used  at  each  station  on  ,   E^. 

long    lines.      It    intro- 

duces  a  local  battery  to 

operate     the    sounder. 

It  is  another  application 

of  the  electromagnet. 


Line 


J=Earth  Earth 

FIG.  133.  —  Diagram  of  simple  telegraph  line. 


MAGNETISM  AND  THE  ELECTROMAGNET  181 

A  diagram  of  a  simple  telegraph  is  shown  in  Fig.  133.  It 
shows  two  stations  X  and  F,  each  equipped  with  a  battery  B, 
key  K,  and  sounder  S.  The  wire  at  each  station  is  grounded 
and  the  earth  serves  as  the  return  wire.  When  no  message  is 
being  sent,  both  keys  are  closed  and  the  current  flows  continu- 
ously. When  one  operator  wishes  to  telegraph,  he  does  so  by 
opening  and  closing  his  key  at  the  right  intervals. 

EXERCISES 

1.  What  is  a  magnet  pole? 

2.  How  would  you  find  the  north  pole  of  a  magnet? 

3.  What  is  magnetic  induction? 

4.  What  is  a  magnetic  line  of  force? 

5.  How  can  you  show  the  lines  of  force  in  the  magnetic  field  of  a 
magnet  ? 

6.  State  the  theory  of  molecular  magnets. 

7.  State  three  facts  about  magnets  which  this  theory  explains. 

8.  Describe  Oersted's  discovery. 

9.  State  the  rule  by  which  we  remember  how  the  north  pole  of  a 
magnetic  needle  turns  when  brought  near  a  wire  carrying  an  electric 
current. 

10.  Define    electromagnet.     Why    is    the    electromagnet    the    most 
important  form  of  magnet? 

11.  State  the  rule  for  finding  the  north  pole  of  an  electromagnet. 

12.  Make  a  diagram  of  an  electric  bell  showing  the  path  of  the  current 
through  the  bell. 

13.  Describe  how  the  current  rings  the  bell. 

14.  Make  a  drawing  showing  a  battery  connected  with  three  bells 
and  three  push  buttons.     In  your  home  trace  the  wires  from  the  battery 
to  each  push  button  and  bell. 

15.  Describe  the  telegraph  sounder. 

1 6.  Make  a  diagram  of  two  telegraph  stations  connected,  each  station 
having  battery,  key,  and  sounder. 


CHAPTER  XVI 


THE   ELECTRIC   MOTOR  IN   THE   HOME 

AN  electric  motor  is  one  of  the  most  useful  of  household  elec- 
trical appliances.     Some  of  the  ways  in  which  it  is  used  are 

shown  in  Fig.  134;  namely,  to 
run  the  sewing  machine,  washing 
machine,  and  wringer.  It  is  also 
used  to  run  the  vacuum  cleaner, 
knife  sharpener,  silver  polisher, 
meat  chopper,  potato  peeler, 
coffee  grinder,  cake  mixer,  egg- 
beater,  ice-cream  freezer,  etc. 

The  kitchen  is  the  workshop 
of  the  home,  and  as  in  other 
workshops,  a  certain  amount  of 
power  is  required  to  do  the  work. 
The  electrical  motor  supplies  this 
power.  Let  us  now  learn  how 
the  electric  current  runs  the 
motor. 

How  does  the  electric  current 
run  a  motor  ?  • —  In  Fig.  135 
we  have  two  straight  magnets 
pivoted  between  the  poles  of  a 
horseshoe  magnet.  We  know 
that  in  this  case  the  straight 
magnets  will  turn  in  the  direction  indicated  by  the  arrows,  be- 
cause each  pole  of  the  horseshoe  magnet  attracts  the  opposite 

182 


FlG.  134.  —  Motor-driven  sewing 
machine  and  washing  machine  and 
wringer. 


THE   ELECTRIC   MOTOR  IN  THE   HOME 


183 


FIG.  135.  —  Illustrat- 
ing the  principle  of 
the  electric  motor. 


poles  of  the  straight  magnets.     The  straight  magnets  stop, 

however,  when  their  north  poles  are  opposite  the  south  pole  of 

the  horseshoe  magnet,  and  their  south  poles 

opposite  the  north  pole.     If  we  had  some 

means  of  reversing  the  poles  of  the  straight 

magnets  when  they  reach  this  position,  they 

would  move  through  another  half  turn.     If 

this  process  of  reversing  the  poles  could  be 

repeated  at  the  proper  intervals,  the  straight 

magnets  would  revolve  continuously.     This 

is  precisely  what  takes  place  in  the  motor. 
The  part  of   a  motor  which  revolves  is 

called  the  armature;   it  is  represented  above  by  the  straight 

magnets.     The  armature  revolves  in  the  magnetic  field  of  a 

large  magnet  called  the  field  magnet.  The  field  magnet  is  repre- 
sented above  by  the  horseshoe  magnet. 
The  diagram,  Fig.  136,  represents  an 
electric  motor  driven  by  a  dynamo. 
D  is  the  dynamo,  very  much  reduced 
in  size.  N  and  S  are  the  north  and 
south  poles  of  the  field  magnet.  The 
parts  n,  n',  sy  s'  are  the  poles  of  the 
magnets  on  the  armature.  All  the 
magnets  in  a  motor  are  electromagnets. 
The  parts  marked  i,  2,  3,  and  4  are 
segments  of  the  commutator.  B  and  B' 
are  the  brushes.  The  commutator  and 
brushes  serve  to  change  the  direction  of 
the  current  in  the  electromagnets  of  the 
armature.  The  direction  of  the  current 
is  changed  twice  hi  each  revolution. 
Let  us  now  see  how  the  electric  current  makes  the  armature 

of  the  motor  revolve.     To  do  this  we  must  review  the  rule  for 

finding  the  poles  of  an  electromagnet.     This  has  already  been 

stated.     It  is :   "  Grasp  the  electromagnet  in  the  right  hand, 


FIG.  136.  —  Diagram  of  an 
electric  motor. 


184  PHYSICS  OF  THE  HOUSEHOLD 

with  the  fingers  pointing  in  the  direction  in  which  the  current 
passes  around  the  magnet ;  the  extended  thumb  then  points  to 
the  north  pole  of  the  magnet." 

The  current  from  the  dynamo  D  enters  the  motor  at  A,  where 
it  divides.  Part  flows  around  the  poles  of  the  field  magnet.  If 
we  use  the  rule  above,  we  find  that  S  is  a  south  pole  and  N  a 
north  pole.  Part  of  the  current  goes  to  the  brush  B,  then  to  the 
segment  of  the  commutator  marked  i.  Here  it  divides  again; 
part  of  it  flows  around  the  electromagnets  n  and  n'  in  such  a  way 
as  to  make  the  ends  marked  n  and  n'  north  poles;  part  of  it 
flows  around  the  electromagnets  s  and  sr  in  such  a  way  as  to 
make  the  ends  marked  s  and  sf  south  poles.  These  two  parts 
of  the  current  unite  on  the  segment  3,  and  leave  the  armature 
by  the  brush  B'.  The  current  then  flows  back  to  the  dynamo. 

Since  n  and  n'  are  north  poles,  they  are  attracted  by  S  and 
repelled  by  N',  also,  since  s  and  sf  are  south  poles,  they  are 
attracted  by  TV  and  repelled  by  S.  These  attractions  and 
repulsions  cause  the  armature  to  revolve  in  the  direction 
indicated  by  the  arrows. 

When  the  armature  has  made  one  quarter  turn,  the  current 
enters  on  segment  4  of  the  commutator  and  leaves  on  segment  2. 
At  each  quarter  turn,  the  segments  on  which  the  current  enters 
and  leaves  the  armature  are  changed.  The  result  is  that  the 
ends  n  and  n'  of  the  electromagnets  on  the  left  of  the 
armature  are  always  north  poles,  and  the  ends  5  and  s' 
of  the  electromagnets  on  the  right  of  the  armature  are  always 
south  poles.  The  armature  thus  revolves  continuously,  as 
long  as  the  current  passes  through  the  motor. 

Commercial  motors.  —  Commercial  motors  are  similar  in 
principle  to  that  shown  in  Fig.  136,  but  differ  in  design.|  The 
motors  used  with  household  appliances  are  almost  always  in- 
closed in  an  iron  casing  to  protect  the  operator  and  also  the 
motor.  The  armature  is  usually  of  the  type  known  as  the  drum 
armature,  Fig.  137.  The  wire  is  wound  lengthwise  on  a  soft 
iron  drum. 


THE  ELECTRIC  MOTOR  IN  THE  HOME 


185 


FIG.   137. — The  drum  armature. 


The  motor  described  above  uses  a  direct  current.  A  direct 
current  is  one  which  flows  in  the  same  direction  all  the  time. 
An  alternating  current  is  one  which  changes  its  direction  many 
times  a  second.  Alternat- 
ing current  motors  are  in 
very  common  use  in  the 
home  and  elsewhere.  They 
are  simpler  in  construction 
than  direct  current  motors ;  but  the  theory  of  their  construc- 
tion is  more  difficult  and  the  study  of  it  would  lead  us  beyond 
the  scope  of  this  book. 

The  motor  in  the  kitchen.  —  A  convenient  power  table  for 
the  kitchen  is  shown  in  Fig.  138.  It  is  equipped  with  a 
\  h.  p.  electric  motor  which  is  connected  with  the  power  head 

by  inclosed  gears.  The 
appliances  to  be  operated 
may  be  connected  with 
the  pulley  on  the  axle  of 
the  motor,  or  with  one 
of  the  two  horizontal 
shafts  which  project 
from  the  power  head. 

/7b/-or-4-4tJ  Wr&==%^Ge<3*s '     There  is  a  slot  in  the  top 

of  the  table  by  means  of 
which  various  appliances 
can  be  fastened  securely 
to  the  table.  In  Fig.  138 
the  power  table  is  shown 
operating  a  bread  mixer. 
A  number  of  attach- 
ments for  the  power  table  are  shown  in  Fig.  139 ;  namely,  the 
bread  mixer,  cake  mixer,  egg  beater,  and  meat  grinder.  In 
addition  the  power  table  can  be  used  to  operate  the  washing 
machine,  wringer,  mangle,  knife  sharpener,  silver  polisher,  coffee 
grinder,  food  chopper,  ice  cream  freezer,  vegetable  slicer,  etc. 


FIG.  138.  —  A  kitchen  power  table. 


i86 


PHYSICS  OF  THE  HOUSEHOLD 


We  have  now  learned  some  of  the  ways  in  which  the  motor 
aids  in  the  work  of  the  home.     New  uses  are  constantly  being 


Cake  Mixer 


Beater 
FIG.   139.  —  Attachments  for  kitchen  power  table. 

found,  and  it  is  probable  that  the  readers  of  this  book  will  be 
able  to  devise  new  ways  of  making  it  of  service  in  the  home. 


EXERCISES 

1.  Make  a  diagram  of  an  electric  motor,  showing  how  the  current 
flows  in  each  part. 

2.  Explain  why  the  armature  of  a  motor  revolves. 

3.  Describe  a  kitchen  power  table.     Name  some  of  the  appliances 
it  will  operate. 


CHAPTER  XVII 


ELECTRIC   HEATING,    COOKING,   AND   LIGHTING 
APPLIANCES 

IN  recent  years  there  have  been  many  electrical  heating  and 
cooking  appliances  placed  on  the  market.  Some  of  these  are 
illustrated  in  Fig.  141 ;  namely,  the  electric  iron,  the  disk 
heater  or  stove,  the  chafing  dish,  coffee  percolator,  electric 
oven,  luminous  radiator,  and  warming  pad,  also  the  current 
regulator  or  rheostat.  Other  common  appliances  are  the  air 
heater,  broiler,  and  toaster. 

The  heating  of  these  appliances  depends  on  the  heating 
effect  of  an  electric  current.  When  an  electric  current  is  sent 
through  any  substance  which  offers  resistance  to  the  current, 
heat  is  produced.  The  rate  at  which  the  heat  is  produced  in- 
creases with  the  current  and  with  the  resistance.  In  most  of 
these  appliances  the  heat  is  produced  by 
sending  an  electric  current  through  a  long 
thin  wire.  This  wire  is  known  as  the 
heating  element. 

The  electric  iron.  —  One  method  of  heat- 
ing an  electric  iron  is  illustrated  in  Fig.  140. 
The  heating  element  (i)  *  consists  of  two 
coils  of  thin  wire  wound  upon  heavy  brass 
cores.  The  current  enters  and  leaves  by 
the  contact  tongues  (2)  and  (3).  The 
current  heats  the  thin  wire  and  this  heat 
is  transferred  by  radiation  and  conduction 
to  the  heavy  bottom  plate  of  the  iron. 

187 


FIG. 
the 
of  an  electric  iron. 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  141.  —  Household  electric  heating  and  cooking  appliances. 


ELECTRIC  APPLIANCES  189 

In  another  type  of  iron  the  heating  element  is  a  long  thin  wire 
baked  in  enamel  to  the  bottom  plate  of  the  iron.  The  current 
heats  the  wire  and  the  enamel  conducts  the  heat  to  the  bottom 
of  the  iron. 

The  disk  stove.  —  The  heating  element  of  the  disk  stove  is 
made  as  follows.  A  long  coil  of  thin  wire  is  wound  in  the  form 
of  a  spiral.  This  is  embedded  in  enamel,  and  the  enamel  is 
baked  to  the  under  side  of  the  top  plate  of  the  stove.  The 
current  heats  the  wife  and  the  heat  is  conducted  by  the  enamel 
to  the  top  of  the  stove. 

The  chafing  dish  and  coffee  percolator.  —  The  heating  ele- 
ment of  these  appliances  is  similar  to  that  of  the  disk  stove 
described  above.  The  lower  part  of  each  appliance  is  a  disk 
stove  over  which  the  upper  part  fits  closely.  They  are  made 
close  fitting  in  order  to  use  the  greatest  possible  amount  of  the 
heat  from  the  stove. 

The  oven  and  luminous  radiator.  —  In  the  oven  there  is  an 
iron  grid  at  the  top  and  another  at  the  bottom.  The  enamel  with 
the  wire  embedded  in  it  is  baked  to  the  top  of  the  upper  grid, 
and  to  the  bottom  of  the  lower  grid.  The  current  heats  the  wire 
and  the  heat  is  conducted  by  the  enamel  to  the  grid  and  then  to 
the  air  in  the  oven.  The  sides  of  the  oven  are  double,  and  the 
space  between  is  filled  with  asbestos  to  retain  as  much  heat  as 
possible. 

The  luminous  radiator  consists  of  a  number  of  large  incan- 
descent lamps  placed  in  front  of  a  reflector.  When  the  current 
is  turned  on,  the  lamps  give  out  heat  and  light. 

The  wanning  pad.  —  The  warming  pad  is  heated  by  passing 
the  current  through  fine  wire.  The  wire  is  made  long  and  thin, 
so  that  it  has  a  high  resistance,  and  therefore  the  current  which 
passes  through  it  is  small  and  the  heat  developed  is  moderate. 
The  wire  is  inclosed  in  asbestos  cloth  to  avoid  any  danger  from 
fire,  and  the  whole  pad  is  covered  with  flannel. 

The  current  is  brought  to  the  pad  by  means  of  a  cord  and  plug, 
which  may  be  attached  to  any  electric  light  socket. 


I  go  PHYSICS  OF  THE  HOUSEHOLD 

The  rheostat  —  The  rheostat  is  an  adjustable  resistance.  It 
controls  the  temperature  of  any  appliance  by  controlling  the 
amount  of  current  which  reaches  it.  It  is  made  of  a  series  of  coils 
of  wire  through  which  the  current  passes. 

When  a  rheostat  is  used  with  any  appliance  it  is  so  arranged 
in  the  circuit  that  the  current  passes  through  the  rheostat  as 
well  as  through  the  appliance.  When  the  sliding  contact  arm 
is  in  the  position  marked  "  low,"  Fig.  141,  the  current  passes 
through  all  the  coils  of  the  rheostat.  The  resistance  in  the  cir- 
cuit is  then  greatest  and  the  current  least.  The  appliance  then 
has  its  lowest  temperature.  When  the  sliding  contact  arm  is 
moved  to  the  right,  some  of  the  coils  of  the  rheostat  are  cut  out, 
and  when  it  reaches  the  position  marked  "  full,"  all  the  coils 
are  cut  out.  The  resistance  in  the  circuit  is  then  least  and  the 
current  greatest.  The  appliance  then  has  its  highest  temper- 
ature. 

The  air  heater,  broiler,  and  toaster.  —  The  heating  element  of 
the  air  heater  is  a  series  of  bare  coils  of  rather  large  iron  wire. 
The  heater  case  is  open  at  the  bottom  and  at  the  top.  When 
the  current  is  turned  on,  the  wires  are  heated  and  an  air  con- 
vection current  is  started.  Cold  air  enters  at  the  bottom  of  the 
case,  is  heated,  and  leaves  as  warm  air  at  the  top. 

The  heating  element  of  the  electric  broiler  is  similar  to  that 
of  the  air  heater,  except  that  the  coils  are  made  of  finer  wire  and 
are  therefore  heated  to  a  higher  temperature  by  the  same 
amount  of  current.  The  meat  to  be  broiled  is  fastened  in  a 
holder  and  placed  in  the  broiler  close  to  the  hot  coils. 

The  heating  element  of  the  toaster  is  a  series  of  bare  coils  of 
thin  wire.  These  coils  are  placed  in  a  horizontal  position 
beneath  a  wire  screen.  The  bread  to  be  toasted  is  placed 
on  this  screen.  The  current  heats  the  wire  coils  and  the  heat 
passes  up  to  the  bread  by  convection  and  radiation. 

We  have  now  examined  a  number  of  electrical  heating  and 
cooking  appliances,  and  we  find  in  each  case  that  the  heating 
element  is  a  wire  which  is  heated  by  an  electric  current.  In  a 


ELECTRIC  APPLIANCES  191 

later  chapter  we  shall  take  up  the  question  of  just  how  the 
quantity  of  heat  produced  per  second  varies  with  the  quantity 
of  current,  amount  of  resistance,  etc. 

ELECTRIC  LIGHTS  IN  THE  HOME 

The  incandescent  light.  —  We  are  all  familiar  with  the  in- 
candescent light.  We  know  that  when  we  turn  on  the  electric 
current,  by  means  of  a  button  or  switch,  the  fine  wire  in  the  bulb 
is  heated  to  the  temperature  at  which  it  gives  white  light.  The 
explanation  of  the  light  from  the  incandescent  lamp  is  the  same 
as  that  which  we  found  to  apply  to  the  heating  and  cooking 
appliances  studied  in  the  preceding  section, 
namely,  when  an  electric  current  is  passed  through 
a  wire  of  high  resistance,  the  wire  is  heated. 

If  we  examine  an  incandescent  light  bulb, 
Fig.  142,  we  find  that  the  part  which  screws  into 
the  socket  consists  of  a  metallic  button  B  in  the 
center,  insulated  from  an  outer  metallic  screw 
cylinder  S.  These  two  metallic  parts  make  FIG.  142.— An 
electrical  contact  with  the  corresponding  parts 
in  the  socket. 

If  we  break  up  an  old  lamp,  we  find  that  each  of  these  metallic 
pieces  is  joined  to  one  of  the  copper  wires  in  the  sealed  glass 
tube  G  which  projects  into  the  bulb.  The  fine  filament  F  in 
the  bulb  of  the  common  incandescent  lamp  is  made  of  carbon 
and  its  ends  are  joined  to  the  copper  wires  in  the  tube  by  means 
of  short  platinum  wires  sealed  into  the  glass.  Platinum  is 
used  for  the  wires  through  the  glass  because  it  expands  and 
contracts  at  the  same  rate  as  the  glass,  and  therefore  does  not 
crack  it  when  the  temperature  changes.  The  filament  is  made 
of  carbon  because  carbon  does  not  melt  easily.  The  tempera- 
ture at  which  it  melts  is  greater  than  that  at  which  it  gives  white 
light.  Carbon,  however,  burns  readily  when  heated  in  air, 
and  to  avoid  this,  the  air  is  pumped  from  the  bulb  and  the  bulb 
is  sealed  air-tight. 


1 92  PHYSICS  OF  THE  HOUSEHOLD 

The  path  of  the  current  through  the  lamp  is  as  follows.  If 
it  enters  at  the  bottom  B,  it  moves  through  a  copper  wire  to  W, 
then  through  platinum  to  G,  then  through  the  filament  F  and 
out  through  the  other  platinum  and  copper  wires  to  the  screw 
cylinder  S.  The  lamp  is  lighted  equally  well  if  the  current 
enters  at  S  and  leaves  at  B.  In  fact,  incandescent  lamps  are 
usually  lighted  by  means  of  an  alternating  current  which  changes 
its  direction  many  times  a  minute. 

Metal  filament  lamps.  —  Recently  the  incandescent  lamp  has 
been  improved  by  replacing  the  carbon  filament  by  one  made  of 
the  metal  tantalum  or  the  metal  tungsten. 

The  advantage  possessed  by  these  metals  is  that  they  will 
stand  a  much  higher  temperature  than  carbon  without  disin- 
tegration, and  the  higher  the  temperature  of  the 
filament  the  greater  is  the  amount  of  light  pro- 
duced from  the  same  current.  For  example, 
with  50  watts  of  electrical  energy  the  carbon 
filament  lamp  produces  a  16  candle-power  light, 
the  tantalum  filament  a  25  candle-power  light 
and  the  tungsten  filament  lamp  a  40  candle- 
power  light. 

Since  electric  current  is  paid  for  at  a  certain 
FIG.  143.  —  The       .  .  ,  ,  .,          ,  ,  .„  , 

tungsten  lamp,     pnce  per  watt  hour  or  kilowatt  hour,  it  will  be 

seen  that  for  the  same  money  2\  times  as  much 
light  is  secured  from  the  tungsten  filament  lamp  as  from  the 
carbon  filament  lamp.  It  is  evident  then  that  in  a  few  years 
all  incandescent  lamps  will  be  of  the  tungsten  filament  type, 
unless  a  lamp  of  higher  light-giving  power  is  produced  in 
the  meantime. 

The  metals  tantalum  and  tungsten  offer  less  resistance  than 
carbon  to  the  electric  current,  and  for  this  reason  the  filaments 
made  of  these  metals  must  be  of  greater  length  to  give  the 
required  amount  of  resistance.  From  the  illustration  (Fig. 
143)  it  will  be  noticed  that  the  tungsten  lamps  have  a  long  fila- 
ment supported  on  a  frame. 


ELECTRIC   APPLIANCES 


193 


FIG.  144. — 
The  arc. 


The  arc  light.  —  The  arc  light  is  used  to  illuminate  streets 
and  large  buildings.     It  is  different  in  principle  from  the  incan- 
descent  light.     We  may  show   how  the   light   is 
produced  as  follows : 

Pass  the  current  from  a  power  circuit  through 
a  rheostat  (to  limit  the  amount  used)  and  then 
through  two  carbon  rods.  If  the  ends  of  the  rods 
are  placed  together  at  first  and  then  pulled  apart 
about  three-eighths  of  an  inch,  a  brilliant  light 
is  produced.  The  ends  of  the  carbon  become 
white  hot,  and  the  space  between  is  filled  with 
small  particles  of  carbon.  The  space  between 
the  carbons  is  called  the  arc  (Fig.  144).  The 
positive  carbon  is  the  brighter  and  is  cupped 
out.  It  is  made  the  upper  carbon  of  the  arc 
light  in  order  to  throw  the  greatest  amount  of  light  downward. 
In  arc  lamps  the  carbons  are  controlled  by  an  electromagnet. 
This  magnet  does  two  things :  first,  when  the  current  is  started, 
it  separates  the  carbons  about  three  eighths  of 
an  inch,  and  forms  the  arc ;  second,  it  keeps 
the  carbons  apart  this  distance. 

The  diagram  (Fig.  145)  illustrates  roughly 
the  working  of  the  electromagnet.  The  current 
enters  the  electromagnet  and  then  passes  down 
through  the  positive  and  negative  carbons  and 
out. 

Before  the  current  is  started  the  carbons  are 
together.  When  the  current  starts,  however, 
M  becomes  a  magnet  and  lifts  one  end  of  the 
iron  bar  shown  below.  The  bar  grips  the  rod 
and  lifts  the  rod  and  the  positive  carbon.  This 
forms  the  arc. 

As  the  current  continues  to  flow,  it  gradually 
wears  away  the  carbons  and  lengthens  the  arc.     The  longer 
the  arc  the  greater  its  resistance  and  therefore  the  less  the 
o 


• 

—  i 

3=r 

M 

4 

] 

\ 

FIG.   145.  —  The 
arc  lamp. 

194  PHYSICS  OF  THE  HOUSEHOLD 

current.  As  the  current  decreases,  M  decreases  in  strength  and 
the  iron  bar  drops  down.  This  allows  the  carbons  to  come 
closer  together.  The  instant  the  carbons  come  closer  together 
the  arc  is  shortened,  its  resistance  is  decreased,  and  therefore 
the  current  increases.  The  magnet  then  becomes  stronger  and 
the  bar  is  drawn  up  again,  etc.  The  magnet  is  so  adjusted 
that  the  carbons  are  kept  about  three  eighths  of  an  inch  apart. 


EXERCISES 

1.  Name  five  electric  cooking  appliances  and  five  electric  heating  appli- 
ances. 

2.  Describe  the  heating  element  of  the  electric  iron,  stove,  oven, 
and  chafing  dish. 

3.  Describe  the  air  heater,  luminous  radiator,   warming  pad,  and 
rheostat.     Make  a  diagram  of  each. 

4.  Make  a  diagram  of  the  carbon  filament  incandescent  lamp.     De- 
scribe it. 

5.  Make  a   diagram  of   the   tungsten   filament  incandescent  lamp. 
Describe  it. 

6.  Describe  an  experiment  to  show  that  an  arc  is  formed  when  a 
strong  current  is  passed  through  two  pieces  of  metal  and  they  are  sep- 
arated a  short  distance. 

7.  Make  a  diagram  of  the  regulating  mechanism  of  the  arc  lamp. 
Describe  it. 

8.  In  your  home,  trace  the  electric  light  wires  from  the  point  they  enter 
the  house  to  each  light  (as  far  as  possible).     Note  the  position  of  the  main 
switch,  meter,  and  fuses.     Read  the  meter  regularly  for  six  months,  and 
with  these  readings  check  the  bills  sent  by  the  electric  light  company. 


CHAPTER  XVIII 
ELECTROPLATING 

MANY  household  utensils  are  plated.  For  example,  spoons, 
forks,  and  table  dishes  may  be  made  of  white  metal,  brass,  or 
Britannia  metal  and  covered  or  plated  with  silver,  that  is,  silver 
plated.  Other  utensils,  such  as  pokers,  stove  lifters,  teakettles, 
metallic  lamps,  etc.,  are  frequently  made  of  iron,  steel,  brass, 
or  copper  and  covered  or  plated  with  nickel.  Many  pieces  of 
jewelry  are  made  of  copper  or  brass,  etc.,  and  then  plated  with 
gold. 

Objects  are  plated  with  copper,  silver,  gold,  etc.,  by  means  of 
an  electric  current.  To  understand  how  this  is  done  we  must 
first  study  electrolysis. 

Electrolysis.  —  It  is  found  by  experiment  that  organic 
liquids,  such  as  gasoline,  alcohol,  kerosene,  etc.,  do  not  conduct 
electricity,  but  that  solutions  of  acids,  bases,  and  salts  do  con- 
duct electricity.  It  is  found  also  that  when  an  electric  current 
passes  through  a  solution  of  an  acid,  base,  or  salt,  the  substance 
in  solution  is  decomposed.  A  solution  which  conducts  elec- 
tricity and  is  decomposed  by  the  current  is  called  an  electrolyte. 
The  process  of  decomposing  a  liquid  by  means  of  an  electric 
current  is  called  electrolysis. 

Electrolysis  of  water.  —  In  Fig.  146,  the  liquid  5  is  water 
containing  a  small  quantity  of  sulphuric  acid.  The  current  is 
passed  through  the  water  by  means  of  strips  of  platinum  at- 
tached to  platinum  wires  sealed  in  the  glass  tubes  G.  After 
the  current  has  passed  through  the  liquid  for  some  time,  we 
find  that  hydrogen  (H)  has  collected  in  one  test  tube  and  oxygen 

195 


196 


PHYSICS  OF  THE  HOUSEHOLD 


(O)  in  the  other.  The  volume  of  hydrogen  is  always  twice 
that  of  the  oxygen,  that  is,  it  is  in  the  proportion  in  which  these 
gases  combine  to  form  water,  H2O.  It  is  found  also  that  the 
H  always  moves  in  the  direction  the  current  moves,  and  is 
deposited  on  the  platinum  plate  by  which  the  current  leaves 
the  liquid.  The  O  moves  in  the  opposite  direction  and  is  de- 
posited on  the  plate  by  which  the  current  enters  the  liquid. 
The  platinum  plate  by  which  the  current  enters  the  liquid  is 

called  the  anode,  the 
plate  by  which  it 
leaves  the  liquid  is 
called  the  cathode. 

If  the  liquid  S  is 
a  solution  of  copper 
sulphate,  CuSO4,  we 
find  that  oxygen  is 
formed  at  the  anode 
as  above,  but  that 
copper  is  deposited  on 
the  cathode  instead  of 
hydrogen. 

The  explanation  of 
electrolysis  is  as  fol- 
lows. When  sulphuric 
acid  is  dissolved  in 

water,  it  breaks  up  into  positively  charged  H  ions  and  nega- 
tively charged  864  ions.  The  SO4  ions  are  attracted  by  the 
positively  charged  anode  and  give  up  their  charge  to  the  anode. 
They  then  unite  with  water  to  form  sulphuric  acid  and  oxygen. 
The  H  ions  are  attracted  by  the  negatively  charged  cathode,  and 
give  up  their  charge  to  it.  They  then  collect  as  hydrogen  gas. 
The  same  explanation  holds  for  other  acids,  and  for  salts  and 
bases.  For  example,  when  the  salt  CuSO4  is  dissolved  in 
water,  it  breaks  up  into  positively  charged  Cu  ions  and  nega- 
tively charged  SOi  ions.  The  SC>4  ions  are  attracted  by  the 


FIG.   146.  —  Electrolysis  of  water. 


ELECTROPLATING 


197 


anode  and  form  oxygen  as  above.  The  Cu  ions  are  attracted 
by  the  cathode  and  form  a  deposit  of  copper  on  it. 

When  a  current  passes  through  a  solution  of  any  salt,  acid, 
or  base,  the  positive,  metal  or  hydrogen,  ions  of  the  solution 
move  with  the  current  and  are  deposited  on  the  cathode.  The 
negative  ions  are  deposited  on  the  anode. 

Electroplating.  —  We  learn  from  our  study  of  electrolysis 
that  when  an  electric  current  is  passed  through  a  solution  of  a 
salt,  the  salt  is  decomposed,  the  metal  part  of  the  salt  moves  in 
the  direction  the  current  moves  and  is  deposited  on  the  cathode. 
This  property  of  the  electric  current  is  used  in  electroplating. 

Copperplating.  —  The  arrangement  of  an  electroplating  outfit 
is  shown  in  Fig.  147.  Let  us  suppose  that  we  wish  to  copper- 
plate a  medal.  We  proceed  as  follows:  Make  a  solution  of  a 


FIG.   147.  —  Electroplating  a  medal. 

copper  salt,  say  copper  sulphate.  Place  in  this  solution  a  plate 
of  pure  copper  and  the  medal  to  be  plated.  Then  connect 
them  with  a  battery  in  such  a  manner  that  the  current  flows, 
first  to  copper  plate,  then  through  the  solution  to  the  medal 
and  back  to  the  battery.  The  medal  receives  a  coating  of 
copper  and  the  copper  plate  loses  an  equal  weight  of  copper. 


198  PHYSICS  OF  THE  HOUSEHOLD 

The  action  is  as  follows:  The  copper  sulphate  (CuSO4)  is 
decomposed  into  copper  ion  (Cu)  and  the  acid  ion  (804). 

Copper  sulphate  =  copper  ion  -f  acid  ion 

CuSO4  Cu        +      SO4 

The  copper  is  deposited  upon  the  medal  and  plates  it.  The 
SO4  is  deposited  upon  the  copper  plate,  and  unites  with  the 
copper  to  form  more  copper  sulphate. 

Copper  -f-  acid  ion  =  copper  sulphate 
Cu      +      SO4     =          CuSO4 

Thus,  in  the  operation  of  plating,  the  medal  receives  a 
certain  weight  of  copper,  the  copper  plate  loses  the  same  weight 
of  copper,  and  the  copper  sulphate  solution  remains  at  a  con- 
stant strength. 

Nickel  plating.  —  If  we  wish  to  nickel  plate  the  medal,  we 
use  a  solution  of  a  nickel  salt  and  a  plate  of  pure  nickel ;  other- 
wise the  arrangement  of  the  apparatus  is  the  same.  Again  the 
solution  remains  at  a  constant  strength,  and  by  weighing  the 
medal  and  nickel  plate  before  and  after  the  plating,  we  find  that 
the  medal  receives  a  certain  weight  of  nickel  and  the  plate 
loses  the  same  weight. 

Silver  and  gold  plating.  —  In  silver  and  gold  plating  the 
arrangement  of  the  apparatus  is  the  same,  except  that  we  use 
different  solutions  and  plates.  In  silver  plating  we  use  a  solu- 
tion of  a  silver  salt  and  lead  the  current  into  the  solution  by  a 
plate  of  silver.  In  gold  plating  we  use  a  solution  of  a  gold 
salt,  and  lead  the  current  into  the  solution  by  a  plate  of  gold. 

The  solution  used  in  silver  plating  is  silver  cyanide  dissolved 
in  potassium  cyanide.  In  gold  plating  the  solution  is  gold 
cyanide  dissolved  in  potassium  cyanide.  In  nickel  plating  the 
solution  is  a  double  salt  of  nickel  sulphate  and  ammonium  sul- 
phate dissolved  in  water.  The  solution  used  in  copper  plating 
is  copper  sulphate  dissolved  in  water. 

Preparation  of  the  object  for  plating.  —  In  preparing  for 
plating  of  any  kind  the  greatest  care  must  be  taken  to  make  the 


ELECTROPLATING  1 99 

object  absolutely  clean.  First  it  is  scoured  and  polished  until 
it  looks  clean.  Then  it  is  attached  to  a  wire  and  dipped  into  a 
hot  solution  of  caustic  potash  or  soda ;  this  takes  off  the  grease. 
It  is  then  washed  in  water,  dipped  into  dilute  acid,  washed  in 
water  again,  and  then  placed  in  the  plating  bath.  An  object 
should  not  be  touched  with  the  fingers  after  coming  from  the 
caustic  potash  solution  because  the  points  so  touched  are 
made  greasy  and  the  deposit  does  not  "  take." 

The  current  used  in  plating  is  of  low  voltage,  and  the  deposit 
is  formed  slowly,  the  usual  time  being  from  twenty-four  to 
forty-eight  hours.  After  the  deposit  is  made,  the  last  step  is 
to  polish  the  object  by  means  of  revolving  wheels  made  of  brass 
wire,  leather,  or  canvas. 

Laws  of  electrolysis.  —  The  laws  of  electrolysis  were  dis- 
covered by  Faraday.  They  are : 

(1)  The  weight  of  any  substance  liberated  by  Ck^giyvn  yiiuttiityL 
of  electricity  is  proportional  to  the  chemical  equivalent  of  that 
substance,  that  is,  to  the  weight  in  grams  of  that  substance 
which  will  replace  i  g.  of  hydrogen,  or  which  will  combine  with 
i  g.  of  hydrogen  to  form  a  chemical  compound. 

(2)  The  weight  of  any  substance  liberated  in  a  given  time  is 
proportional  to  the  quantity  of  electricity  which  passes  in  that 
time.    That  is,  the  weight  liberated  is  proportional  to,  amperes 
X  seconds. 

(3)  The  energy  of  the  electrolytic  action  is  the  same  in  all  parts 
of  the  circuit.     For  example,  if  a  current  passes  in  series  through 
a  silver-plating  bath,  a  gold-plating  bath,  and  a  copper-plating 
bath,  then  the  current  which  liberates  one  chemical  equivalent 
of  silver  also  liberates  one  chemical  equivalent  of  gold,  and  one 
chemical  equivalent  of  copper. 

Storage  cells.  —  A  storage  cell  differs  from  the  electric  cells 
we  studied  in  chapter  XIV,  in  that  the  plates  are  both  of  the  same 
metal,  namely,  lead.  These  lead  plates  are  perforated  and  the 
holes  are  filled  with  compressed  oxide  of  lead,  Fig.  148.  The 
liquid  between  the  plates  is  dilute  sulphuric  acid.  When  a 


200 


PHYSICS  OF  THE  HOUSEHOLD 


current  is  passed  through  the  cell,  hydrogen  is  deposited  on  the 
cathode  and  oxygen  on  the  anode,  just  as  they  are  in  the  elec- 
trolysis of  water.     The  hydrogen  acts  on  the  lead  oxide  con- 
tained in  the  holes  of  the  cathode,  and  reduces  it  to  spongy  lead. 
The  oxygen  oxidizes  the  lead  oxide  on  the  anode,  and  turns  it 
into  lead  peroxide.     When  the  storage  cell  is  charged,  we  have 
in  reality  two  different  plates,  one  of  lead,  and  one  of  lead  per- 
oxide.   If  the  cell  now 
is  connected  with  an 
electrical  appliance,  it 
gives  an  electric  cur- 
rent in   the  opposite 
direction  to  the  cur- 
rent which  charged  it. 
This  current  will  con- 
tinue  until   the   lead 
plate   is   oxidized    to 
lead   oxide,   and    the 
lead  peroxide  plate  is 
reduced  to  lead  oxide. 
The  plates   are   then 
identical,  and  the  cur- 
rent ceases.     The  cell 
may    then    be    re- 
charged, when  it  will 
again  give  a  current; 
and  so  on. 

It  will  be  noticed  that  the  cell  stores  up  not  electricity  but 
chemical  energy. 

The  commercial  storage  cells  are  usually  made  up  of  a  number 
of  plates.  For  example,  the  cell,  Fig.  148,  has  five  negative 
plates  and  six  positive  plates.  All  the  negative  plates  are 
joined  to  the  negative  terminal,  and  all  the  positive  plates  to 
the  positive  terminal.  The  storage  cell  gives  an  electromotive 
force  (e.  m.  f.)  of  a  little  over  two  volts,  and  since  the  plates 


FIG.  148.  — The  storage  cell. 


ELECTROPLATING 


201 


are  large  and  close  together,  the  resistance  is  small  and  there- 
fore the  rate  of  flow  of  current  in  amperes  is  large.  These  cells 
have  an  efficiency  of  about  75  per  cent ;  that  is,  they  give  back 
75  per  cent  of  the  electrical  energy  used  in  charging  them. 

The  action  of  the  storage  cell  can  be  illustrated  by  a  simple 
experiment  as  follows:  Place  two  strips  of  sheet  lead,  A  and 
B  (Fig.  149),  in  a  dilute  solu- 
tion of  sulphuric  acid  (i  part 
acid  to  40  parts  water)  and 
connect  these  with  two  storage 
cells  joined  in  series. 

It  will  be  noticed  that  the 


FIG.  149.  —  A  simple  storage  cell. 


plate  on  which  the  current 
enters,  the  anode  A,  becomes 
coated  with  a  red  substance; 
this  is  lead  peroxide.  Hydro- 
gen bubbles  rise  from  the 
cathode  B.  If  after  the  charg- 
ing current  has  been  running  for  five  or  ten  minutes  the  plates 
are  connected  with  an  electric  bell,  the  storage  cell  rings  the 
bell. 

If  the  direction  of  the  charging  current  and  of  the  discharging 
current  are  determined  by  means  of  a  galvanometer,  it  will  be 
found  that  they  flow  in  opposite  directions. 


EXERCISES 

1.  Define  electrolysis,  electrolyte,  anode,  cathode. 

2.  What  liquids  conduct  electricity  readily?     What  liquids  do  not? 

3.  Describe  what  happens  when  an  electric  current  is  passed  between 
platinum  plates  dipped  in  a  solution  of  H2SO4,  of  CuSC>4. 

4.  Make  a  diagram  illustrating  the  arrangement  of  apparatus  for 
copper  plating,  for  nickel  plating.     Describe  what  takes  place  in  each. 

5.  What  solution  is  used  in  silver  plating,  in  gold  plating,  in  nickel 
plating,  in  copper  plating? 

6.  State  the  laws  of  electrolysis. 

7.  Describe  the  storage  cell. 


CHAPTER  XIX 
ELECTRICAL  TERMS  AND   MEASURES 

IN  this  chapter  we  shall  study  the  meaning  of  such  electrical 
terms  as  volt,  ohm,  ampere,  watt,  etc. 

If  we  examine  an  electric  iron,  we  notice  a  small  metallic 
plate  stamped  something  like  this  :  no  V,  4  A,  440  W, 
which  means  that  the  iron  is  to  be  used  on  a  no- volt  circuit, 
that  it  uses  4  amperes  current,  and  that  the  rate  at  which  it 
uses  electrical  energy  is  440  watts.  An  electric  stove  may  be 
marked  as  follows:  no  V,  2  A,  220  W,  which  means  that  it 
is  to  be  used  on  a  no- volt  circuit,  that  it  uses  2  amperes  cur- 
rent, and  that  it  uses  electrical  energy  at  the  rate  of  220  watts. 

Practically  every  electrical  appliance  is  stamped  with  similar 
letters  and  figures,  and  when  we  understand  their  meaning,  we 
know  at  once  the  electrical  conditions  under  which  the  appli- 
ance will  work. 

Common  electrical  terms.  —  The  terms  in  most  common  use 
in  connection  with  the  electric  current  are  ampere,  ohm,  volt, 
watt,  watt  hour,  kilowatt,  and  kilowatt  hour.  Two  other 
terms  which  we  shall  meet  less  frequently  are  coulomb  and  joule. 
These  terms  are  all  derived  from  the  names  of  distinguished 
scientists,  and,  therefore,  do  not  in  themselves  give  us  any 
hint  of  their  meaning. 

The  ampere  is  the  unit  of  current  strength.  It  is  the  strength 
of  current  which  when  made  to  deposit  silver  by  electrolysis 
will  deposit  .001118  g.  of  silver  per  second. 

The  ohm  is  the  unit  of  resistance.  It  is  defined  as  the  resist- 
ance at  o°  C.  of  a  column  of  mercury  106.3  cm-  l°ng>  and  of  uni- 
form cross  sectional  area  of  i  sq.  mm. 

202 


ELECTRICAL  TERMS  AND   MEASURES  203 

The  volt  is  the  unit  of  electrical  pressure  or  electromotive  force. 
It  is  the  electromotive  force  which  will  force  a  one  ampere 
current  through  a  circuit  having  a  resistance  of  one  ohm. 

The  watt  is  the  unit  of  electrical  power.  It  is  the  power  or 
rate  of  working  of  a  current  of  one  ampere  driven  by  an  elec- 
tromotive force  of  one  volt. 

RELATIONSHIPS  AMONG  ELECTRICAL  UNITS 

Ohm's  law.  —  One  of  the  most  important  laws  relating  to 
the  electric  current  is  Ohm's  law.  It  is  :  The  current  in  any 
circuit  is  directly  proportional  to  the  electromotive  force  and  in- 
versely proportional  to  the  resistance. 

A  simpler  way  of  stating  the  same  thing  is  as  follows  :  Ohm's 
law  is: 

Current  in  amperes  =      e.  m.  f.  in  volts 
resistance  in  ohms 

Or,  if  we  let  :  C  =  current  in  amperes,  E  =  the  e.  m.  f.  in  volts, 
and  R  =  the  resistance  in  ohms,  Ohm's  law  is  : 


Ohm's  law  states  the  relation  which  the  ampere,  volt,  and 
ohm  bear  to  one  another. 

The  definition  of  the  watt  gives  us  the  relation  of  the  watt 
to  the  ampere,  volt,  and  ohm.  The  power  of  any  current  in 
watts  is  found  by  multiplying  the  current  in  amperes  by  the 
e.  m.f.  in  volts. 

Power  in  watts  =  current   in   amperes  X  electromotive   force 
in  volts. 

Or  if  we  let  :  W  =  watts,  C  =  current  in  amperes,  and  E  = 
e.  m.  f  .  in  volts,  we  have  the  equation  : 

W  =  CXE 

This  expresses  the  relation  between  watts,  amperes,  and 
volts. 


204  PHYSICS  OF  THE  HOUSEHOLD 

We  may  find  the  relation  between  watts,  amperes,  and  ohms, 
or  between  watts,  ohms,  and  volts  by  means  of  Ohm's  law 
and  the  equation  W  =  CE. 

By  Ohm's  law  :  C  =  -,  or  CR  =  E 

Tf 

If  in  the  equation  W  —  CE,  we  substitute  for  C,  its  value  —  , 

R 

we  obtain  the  equation  : 


which  expresses  the  relation  between  watts,  volts,  and  ohms. 

If,  in  the  equation  W  =  CE  we  substitute  for  E  its  value  CR, 
we  have  the  equation  : 

W  =  C2R 

which  expresses  the  relation  between  watts,  amperes,  and  ohms. 

The  heating  effect  of  a  current.  Joule's  law.  —  The  heat- 
ing effect  of  an  electric  current  was  carefully  investigated  by 
the  English  scientist  Joule,  who  found  that  when  the  power  of 
a  current  is  i  watt,  and  when  all  the  electrical  energy  of  the 
current  is  used  to  produce  heat,  .24  gram  calories  of  heat  are 
produced  each  second. 

That  is,  a  current  with  a  power  of  i  watt  produces  .24  gram 
calories  of  heat  each  second,  or 

Calories  =  watts  X  .24  X  time  in  seconds. 

In  the  last  paragraph  we  found  three  equations,  each  of 
which  describes  a  method  of  finding  the  power  of  a  current  in 
watts;  namely, 

W  =  CE 


W  =  C2R 

We  can  calculate  the  heat  produced  in  any  electrical  device 
when  we  know  any  two  of  the  three  quantities  C,  E,  R,  where 
C  is  the  current  in  amperes  which  flows  through  the  device, 


ELECTRICAL  TERMS   AND   MEASURES  205 

E  is  the  e.  m.  f.  necessary  to  force  the  current  C  through  it, 
and  R  is  the  resistance  of  the  device. 

If  H  =  the  heat  produced  in  gram  calories  and  i  —  the 
time  in  seconds, 

E  =  CEt  X  .24 

or  H  =  ~  /  X  .24 

R 

or  H  =  C2Rt  X  .24 

Any  one  of  these  equations  may  be  used  to  calculate  the  heat 
produced  in  any  electrical  device.  Joule  expressed  the  results 
of  his  experiments  in  the  form 

H=  C*Rt  X  .24 

This  equation  states  that  the  heat  produced  is  proportional 
to  the  time,  to  the  resistance,  and  to  the  square  of  the  current. 
This  is  known  as  Joule's  law. 

We  have  now  learned  a  number  of  equations  which  state  the 
relation  between  amperes,  volts,  ohms,  watts,  and  gram  calories. 
The  three  in  most  common  use  are 

Ohm's  law  C  =  - 

Watts,  W  =  CE 

Joule's  law,  H  =  C*Rt  X  .24 

ELECTRICAL  TERMS  APPLIED  TO  COMMON  APPLIANCES 

We  shall  be  able  to  remember  the  terms,  ampere,  volt,  ohm, 
and  watt  more  readily  if  we  associate  them  with  electrical  ap- 
pliances with  which  we  are  familiar. 

Volt.  —  The  electromotive  force  of  the  Daniell  cell  is  about 

1  volt,  of  the  dry  cell  1.5  volts,  and  of  the  storage  cell  about 

2  volts. 

Until  the  dynamo  was  invented  high  voltages  could  be 
obtained  only  by  connecting  a  number  of  cells  in  series.  For 
example,  100  Daniell  cells  connected  in  series  give  an  e.  m.  f. 
of  100  volts.  With  the  dynamo,  however,  high  voltages  are 


206  PHYSICS  OF  THE  HOUSEHOLD 

easily  obtained.  The  lighting  current  in  homes  has  usually  an 
e.  m.  f.  of  no  volts  or  220  volts.  The  current  used  to  drive 
street  cars  has  usually  an  e.  m.  f .  of  500  volts.  These  illustra- 
tions will  help  us  to  remember  the  meaning  of  the  term  wit. 

Ampere.  —  The  current  strength  obtained  from  a  Daniell 
cell,  when  the  copper  and  zinc  plates  are  joined  by  a  wire  of 
small  resistance,  is  about  i  ampere;  from  the  ordinary  dry 
cell  about  15  amperes.  The  current  used  in  an  ordinary  incan- 
descent light  on  a  no- volt  circuit  is  about  \  ampere  and  on  a 
2  20- volt  circuit  about  J  ampere.  The  motor  used  to  run  the 
sewing  machine,  washing  machine,  etc.,  uses  about  i  ampere 
current.  The  current  used  in  arc  lighting  is  about  10  amperes. 

The  ohm.  —  The  resistance  of  the  ordinary  Daniell  cell  is 
about  i  ohm,  and  of  the  dry  cell  about  .1  ohm.  The  resistance 
of  a  1 6  candle-power  incandescent  lamp  constructed  for  use  on 
a  no-volt  circuit  is  about  220  ohms.  The  No.  22  copper  wire 
which  we  use  in  the  laboratory  to  connect  battery  with  bell, 
etc.,  has  a  resistance  of  i  ohm  for  every  62  ft. 

Watt.  —  The  Daniel  cell  has  an  e.  m.  f.  of  i  volt  and  gives 
a  current  of  about  i  ampere,  therefore  since  watts  =  volts 
X  amperes,  it  works  at  the  rate  of  i  X  i  =  i  watt.  A  dry 
cell  has  an  e.  m.  f.  of  1.5  volts,  and  when  it  is  giving  a  current 
of  15  amperes  it  is  working  at  the  rate  of  1.5  X  15  =  22.5 
watts.  A  1 6  candle-power  incandescent  lamp  on  a  no-volt 
circuit  uses  \  ampere  current.  The  rate  at  which  it  consumes 
electrical  energy  is  no  X  \  =  55  watts.  An  iron  which  uses 
4  amperes  current  on  a  no-volt  circuit  is  using  energy  at  the 
rate  of  no  X  4  or  440  watts. 

These  examples  will  help  us  to  understand  the  meaning  of 
the  four  important  electrical  terms,  wit,  ampere,  ohm,  watt. 

Kilowatt,  watt  hour,  kilowatt  hour,  coulomb,  joule.  — 
The  terms  kilowatt,  watt  hour,  and  kilowatt  hour  are  common 
electrical  terms.  The  kilowatt  simply  means  1000  watts. 
An  applicance  which  is  using  electrical  energy  at  the  rate  of 
1000  watts  is  said  to  be  using  it  at  the  rate  of  i  kilowatt.  The 


ELECTRICAL  TERMS   AND   MEASURES  207 

terms  watt  hour  and  kilowatt  hour  are  used  to  denote  the 
amount  of  energy  used  by  an  electrical  appliance.  An  appli- 
ance which  uses  energy  at  the  rate  of  i  watt  for  one  hour  is 
said  to  use  i  watt  hour  of  energy  ;  if  it  uses  energy  at  the  rate 
of  i  kilowatt  for  one  hour,  it  uses  i  kilowatt  hour  of  energy. 

The  terms  coulomb  and  joule  are  less  often  met.  The  cou- 
lomb is  the  unit  of  quantity  of  electricity.  In  a  circuit  in  which 
a  current  of  i  ampere  is  flowing,  i  coulomb  of  electricity  passes 
any  point  in  the  circuit  in  one  second.  The  joule  is  the  unit  of 
electrical  work.  A  current  working  at  the  rate  of  i  watt  does 
i  joule  of  work  per  second. 

ELECTRICAL  CALCULATIONS 

Ohm's  law.  —  Problem.  —  A  dry  cell  has  an  e.  m.  f.  of  1.5  volts  and 
gives  a  current  of  15  amperes  when  its  plates  are  joined  by  a  wire  of  negli- 
gible resistance.  What  is  the  resistance  of  the  cell  ? 

Tf 

Answer.  —  Ohm's  law  is  C  =  —  ,  that  is 
Amperes 


ohms 

/.  15  =  ^  or  15  R  =  1.5,  or  R  =  ±$ 
R  15 

.-.R  =  .1 

The  resistance  of  the  cell  is  .1  ohm. 

Problem.  —  An  incandescent  lamp  used  on  a  no-volt  circuit  has  a 
resistance  of  220  ohms.  What  current  passes  through  the  lamp? 

Ohm's  law  is,  amperes  =  ^^- 
ohms 

/.  amperes  =  |£$  =  \ 

The  current  passing  through  the  lamp  is  \  ampere. 

Problem.  —  The  heating  element  of  an  electric  iron  has  a  resistance 
of  no  ohms.  What  e.m.f.  will  be  necessary  to  force  a  current  of 
2  amperes  through  the  iron? 

Ohm's  law  is,  amperes  —  vo 


ohms 
2  = 
The  e.m.f.  necessary  is  220  volts. 


,  or  E  =  220 

1 10 


208  PHYSICS  OF  THE  HOUSEHOLD 

Watts.  —  Problem.  —  An  electric  iron  uses  a  current  of  2  amperes 
on  a  22o-volt  circuit.  Find  the  rate  in  watts  at  which  it  uses  electrical 

energy. 

W  =  C  X  E,  or  watts  =  amperes  X  volts 

.'.  watts  =  2  X  220  =  440 

The  iron  uses  electrical  energy  at  the  rate  of  440  watts. 
Problem.  —  The  metal  plate  on  an  electric  stove  is  marked  no  V., 
1 100  W.     Find  the  current  it  uses. 

Watts  =  amperes  X  volts 
1100=  amperes  X  no 

I  TOO 

/.amperes  = =  10 

no 

The  stove  uses  10  amperes  current. 

Problem.  —  An  electric  oven  is  marked  noo  W.,  5  A.     What  e.m.f. 

is  required  ? 

Watts  =  amperes  X  volts 

noo  =  5  X  volts,  or  volts  =  220 

The  e.  m.  f.  required  is  220  volts. 

Joule's  law.  —  Problem.  —  The  heating  element  in  an  electric  iron 
has  a  resistance  of  110  ohms,  and  the  current  passing  through  the  iron 
is  2  amperes.  How  much  heat  is  produced  in  the  iron  in  one  minute? 

Joule's  law  is  H  =  C2Rt  x  .24,  that  is 

Gram  calories  =  (amperes)2  X  ohms  X  seconds  X  .24 
or  H  =  4  X  1 10  X  60  X  .24. 

H  =  6336. 

There  are  6336  gram  calories  of  heat  produced  in  the  iron  in  one 
minute. 

A  gram  calorie  is  the  amount  of  heat  required  to  heat  i  g.  of  water 
i°  C.  A  kilogram  calorie  is  the  amount  of  heat  required  to  heat  i  kg. 
(1000  g.)  of  water  i°  C. 

To  find  the  number  of  kilogram  calories  of  heat  produced  per  second 
in  any  electrical  device.  Joule's  law  is  expressed 

H  (kilogram  calories)  =  C^Rt  X  '24 

IOOO 

or  H  =  C2Rt  X  .00024 

In  English-speaking  countries  the  common  unit  of  heat  is  the  British 
Thermal  Unit  (B.T.U.). 

The  British  Thermal  Unit  is  the  amount  of  heat  required  to  heat 
i  Ib.  of  water  i°  F.  It  has  been  found  that 

i  watt  =  .0009478  B.T.  U.  per  second 
.'.  H  (B.T.  U.)  =  ORt  X  .0009478 


ELECTRICAL  TERMS  AND   MEASURES  2OQ 

Problem.  —  The  heating  element  of  an  electric  iron  has  a  resistance 
of  no  ohms.  The  current  used  is  4  amperes.  How  many  kilogram 
calories  of  heat  are  produced  per  minute? 

//  (kilogram  calories)  =  4  X  no  X  60  X  .00024  =  6.336 

The  heat  produced  in  one  minute  is  6.336  kilogram  calories. 

Problem.  —  The  heating  element  of  an  electric  oven  has  a  resistance 
of  44  ohms.  The  current  is  5  amperes.  How  many  B.  T.  U.  of  heat 
are  produced  in  one  minute? 

B  (B.T.U.)  =  C2Rt.  X  .0009478 

=  25  X  44  X  60  X  .0009478 
=  62.5 

There  are  62.5  British  Thermal  Units  produced  in  one  minute. 
Electrical  heating  and  cooking  appliances  are  usually  marked  with 
the  number  of  watts  of  energy  they  use.     In  this  case  the  amount  of 
heat  produced  may  be  calculated  directly. 

Problem.  —  An  electric  oven  uses  electrical  energy  at  the  rate  of 
1 1 oo  watts.  How  many  gram  calories  of  heat  are  produced  in  it  per 
minute? 

H  (gram  calories)  =  Watts  X  seconds  X  .24 

=  1 100  X  60  X  .24  =  15,840 

There  are  15,840  gram  calories  of  heat  produced  per  minute. 
Kilowatt  hours.  —  Problem.  —  A  6-lb.  electric  iron  is  using  5  amperes 
current  on  loo-volt  circuit.     How  many  kilowatt  hours  of  energy  does 
it  use  in  8  hr.  ? 

Watts  =  amperes  X  volts 
W  =  5  X  ioo  =  500 

The  iron  is  using  energy  at  the  rate  of  500  watts. 

A  kilowatt  is  1000  watts,  therefore  the  iron  is  using  energy  at  the 
rate  of  500  divided  by  1000  or  \  kilowatt. 

In  8  hr.  the  iron  uses  8X5  =  4  kilowatt  hours  of  energy. 

Cost  of  electric  heating.  —  Problem.  —  The  cost  of  electric  energy 
is  usually  about  10  ct.  per  kilowatt-hour.  What  is]  the  cost  of  the 
current  used  in  the  iron  above  in  8  hr.? 

The  iron  uses  4  kilowatt  hours  of  energy  in  8  hr.  The  cost  of 
electric  energy  is  10  ct.  per  kilowatt  hour,  therefore  the  cost  of  the 
electric  energy  is  10  X  4  =  40  ct. 

Problem.  —  An  electric  oven  is  marked  1500  watts.  With  electric 
energy  at  10  ct.  per  kilowatt  hour,  what  is  the  cost  of  baking  for  2  hr  ? 

1500  watts  =  1.5  kilowatts.     In   2    hr.  the  oven   uses    1.5  X  2  =3 
kilowatt  hours  of  energy.     The  cost  is  3  X  10  =  30  ct. 
p 


210  PHYSICS  OF  THE  HOUSEHOLD 

Horse  power  of  an  electric  current.  —  In  English-speaking  countries 
we  measure  the  power  of  engines,  such  as  a  steam  engine,  gasoline  en- 
gine, etc.,  in  horse  power,  and  in  order  to  compare  the  power  of  anelec- 
trical appliance,  such  as  the  motor,  with  these  engines,  we  must  know 
the  relation  between  the  watt  and  horse  power.  This  has  been  deter- 
mined, and  it  is  found  that 

746  watts  =  i  horse  power 

Knowing  this  relation,  we  can  calculate  the  horse  power  of  any  motor. 
For  example,  a  sewing  machine  motor  on  a  2  20- volt  circuit  uses  about 
I  ampere. 

watts  =  220  X  i  =  55  and  the  horse  power  =  7\5^  =  .07  h.p. 

or  about  T?  n-  P- 

EXERCISES 

1.  Define  ampere,  ohm,  volt,  watt. 

2.  State  Ohm's  law. 

3.  How  is  the  power  of  a  current  determined  in  watts? 

4.  State  Joule's  law. 

5.  State  the  e.  m.  f .  in  volts  of  the  current  in  some  common  appliances. 

6.  State  the  rate  of  current  in  amperes  obtained  from  different  cells, 
and  of  the  current  used  in  common  appliances. 

7.  State  the  resistance  in  ohms  of  some  common  appliances. 

8.  State  the  power  in  watts  of  the  current  obtained  from  different 
cells,  and  of  the  current  used  in  some  common  appliances. 

9.  Define  kilowatt,  watt  hour,  kilowatt  hour,  coulomb,  and  joule. 

10.  If  a  Daniell  cell  has  an  e.  m.  f.  of   i  volt,  and  gives  a  current  of 
\  ampere,  when  its  plates  are  joined  by  a  wire  of  negligible  resistance, 
what  is  the  resistance  of  the  cell  in  ohms? 

11.  A  tungsten  lamp  on  a  no- volt   circuit  has  a  resistance  of  440 
ohms.     What  current  does  it  use? 

12.  What  e.  m.  f .  will  be  necessary  to  force  a  current  of   2   amperes 
through  the  heating  element  of  an  electric  iron  which  has  a  resistance 
of  55  ohms? 

13.  An  immersion  coil  is  marked  500  watts.     It  is  used  on  a  no-volt 
circuit.     What  current  in  amperes  flows  through  it? 

14.  A  curling  iron  heater  on  a  no-volt  circuit  uses  \  ampere  current. 
What  is  the  rate  in  watts  at  which  it  uses  electrical  energy? 

15.  A  coffee  percolator  is  marked  350  watts;   it  is  used  on  a  no- volt 
circuit.     What  current  does  it  use? 

16.  A  i  h.  p.  motor  (746  watts)  is  used  on  a  no-volt  circuit.     What 
current  does  it  use  ? 


ELECTRICAL  TERMS  AND   MEASURES  211 

17.  An  electric  iron  is  marked  475  watts.     If  all  this  electrical  energy 
is  turned  into  heat,  how  many  gram  calories  are  developed  per  minute  ? 

18.  An  electric  toaster  has  a  resistance  of  40  ohms,  and  uses  3  am- 
peres current.     How  many  gram  calories  are  produced  in  it  per  minute  ? 

19.  An  immersion  coil  is  marked  500  watts.     How  many    British 
Thermal  Units  are  produced  in  it  per  minute? 

20.  An  immersion  coil  is  marked  500  watts.     How  long  will  it  take 
it  to  heat  i  imperial  quart  of  water  (2^  Ib.)  from  60°  F.  to  212°  F.? 

21.  If  electric  energy  costs  10  cents  per  kilowatt  hour,  what  is  the 
cost  of  using  each  of  the  following  for  5  hr. :   an  iron  marked  400  watts ; 
a  luminous  radiator  marked  500  watts;    an  oven  marked  1500  watts; 
an  air  heater  marked  2500  watts? 

22.  A  sewing  machine  motor  uses  \  ampere  current  on  a  no-volt 
circuit.     What  is  the  horse  power  of  the  motor? 

23.  If  electrical  energy  costs  10  cents  per  kilowatt  hour,  what  is  the 
cost  of  the  energy  required  to  run  the  motor  (Exercise  22),  for  10  hours? 
for  i  hour? 

24.  A  kitchen  motor  uses  i  ampere  current  on  a  loo-volt  circuit.     If 
electrical  energy  costs  10  cents  per  kilowatt  hour,  what  is  the  cost  of  the 
energy  required  to  run  the  motor  for  i  hr  ? 


CHAPTER  XX 

MEASURING  INSTRUMENTS.     SERIES  AND  PARALLEL 
CONNECTIONS 

Electrical  measuring  instruments.  —  Instruments  used  to 
measure  electric  current  are  called  galvanometers.  The  first 
part  of  the  name  is  from  Galvani,  the  name  of  the  Italian  scien- 
tist who,  in  1786,  discovered  that  a  continuous  electric  current 
could  be  produced  by  chemical  action.  For  many  years  the 
electric  current  was  called  the  galvanic  current.  The  word 
"  meter  "  means  "  to  measure,"  and  galvanometer  means  a 
measurer  of  the  electric  current. 

There  are  two  types  of  galvanometers,  each  of  which  consists 
of  a  coil  of  wire  and  a  magnet.  In  galvanometers  of  one  type 
the  magnet  moves,  and  the  coil  remains  stationary.  In  those 
of  the  other  type,  the  coil  moves,  and  the  magnet  remains 
stationary. 

In  our  experiments  on  the  magnetic  effect  of  the  electric 
current,  page  175,  we  found  that  if  a  magnetic  needle  is  placed 
near  a  piece  of  wire  and  a  current  is  sent  through  the  wire,  the 
magnetic  needle  tends  to  turn  at  right  angles  to  the  wire.  If 
the  wire  passes  under  and  over  the  needle,  the  turning  force  is 
doubled ;  and  if  the  magnetic  needle  is  placed  in  the  center  of 
a  coil  of  many  turns  of  wire,  the  turning  force  is  increased 
approximately  in  proportion  to  the  number  of  turns  of  wire  in 
the  coil.  Galvanometers  of  one  type,  Fig.  150,  are  made 
in  this  way :  a  small  magnetic  needle  is  suspended  in  the 
center  of  a  coil  of  many  turns  of  wire.  When  a  current  is  sent 
through  the  wire,  the  magnetic  needle  is  deflected  and  the 

212 


MEASURING   INSTRUMENTS 


213 


FIG. 


150.  — A  galvanometer  with  fixed  coil  and 
movable  magnet. 


amount  of  the  deflection  is  used  to  determine  the  strength  of 

the  current. 
The  second  type  of  galvanometer,  known  as  the  D' Arson val 

galvanometer,  consists  of  a  coil  of  insulated  wire  wound  on  a 

frame,  about  a  piece 

of    soft    iron,    and 

suspended  between 

the  poles  of  a  per- 

man[e!nt    magnet, 

Fig.  151.     A  coil  of 

insulated      wire 

wound  about  a  piece 

of   soft   iron   is  an 

electromagnet. 

This  electromagnet 

is  suspended  so  that 

its  poles  are  at  right  angles  to  those  of  the  permanent  magnet. 

When  a  current  is  passed  through  the  coil,  it  is  magnetized, 
and  tends  to  turn  its  poles  toward  the  poles 
of  the  permanent  magnet.  The  amount  of 
this  turning  is  a  measure  of  the  strength  of 
the  electric  current.  The  coil  is  suspended 
by  means  of  a  fine  ribbon  of  copper  wire 
which  leads  the  current  into  the  coil.  The 
current  is  led  away  from  the  coil  by  means 
of  a  fine  metallic  spring  below  the  coil. 
When  the  coil  turns,  it  twists  the  fine 
copper  ribbon,  and  the  elastic  force  tend- 
ing to  untwist  the  ribbon  acts  against  the 
magnetic  force  turning  the  coil.  The 
mirror  attached  to  the  coil  is  used  to 


FIG.  151.  —  A  D'Arson- 
val  galvanometer  with 


fixed      magnet 
movable  coil. 


1  measure  the  current,  as  follows.  A  scale 
and  telescope  are  so  placed  that  an  image 
of  a  small  part  of  the  scale  can  be  seen  in  the  mirror  by  means 
of  the  telescope.  When  the  mirror  moves,  another  part  of  the 


214 


PHYSICS  OF  THE  HOUSEHOLD 


scale  is  brought  into  view.     The  difference  between  the  two 
scale  readings  serves  as  a  measure  of  the  current. 

Commercial  measuring  instruments.  —  The  voltmeter,  Fig. 
152,  is  used  to  measure  the  e.  m.  f.  of  a  current  in  volts.  The 
ammeter,  Fig.  153,  is  used  to  measure 
the  strength  of  a  current  in  amperes. 
These  instruments  are  both  D' Arson val 
galvanometers.  In  each  a  coil  of  high 
resistance  is  wound  on  a  frame  about  a 
piece  of  soft  iron  and  placed  between  the 
poles  of  a  strong  permanent  magnet. 
This  coil  is  on  pivots  and  is  held  in  posi- 
tion by  a  fine  spring.  The  current  turns 
FIG.  152.— The  voltmeter,  the  coil  against  the  force  of  this  spring. 
In  each  there  is  a  second  coil  through 

which  part  of  the  current  passes.  In  the  ammeter  this  second 
coil  has  a  low  resistance  and  therefore  carries  the  greater  part 
of  the  current.  In  the  voltmeter  the  coil  has  a  high  resistance. 
If  you  use  electricity  in  your  homes,  you  will  find  an  electric 
meter  near  the  point  at  which  the  current  enters  the  house. 
This  meter  measures  the  electrical  energy 
used,  in  kilowatt  hours,  and  it  is  called  a 
kilowatt-hour  meter.  It  is  simply  a  small 
light-running  motor  which  uses  a  small 
part  of  the  current  to  turn  the  hands  on 
the  dial.  This  dial  registers  the  number 
of  kilowatt  hours  of  energy  used  from  one 
visit  of  the  inspector  to  the  next. 

Service  entrance,  fuses,  and  meter  connection.  —  In  Fig. 
154  is  represented  the  method  of  bringing  wires  into  the  house, 
or  the  service  entrance.  The  two  wires  from  the  poles  are 
attached  to  the  side  of  the  house.  The  wires  then  form  a  drip 
loop  and  pass  through  the  wall  through  two  porcelain  tubes 
sloping  upward  toward  the  inside.  The  loop  allows  rain 
water  to  drip  from  the  wires,  and  the  upward  slant  of  the  por- 


FIG.  153.— The 
ammeter. 


MEASURING   INSTRUMENTS 


215 


celain  tubes  excludes  water  from  the  tubes.    Sometimes,  for  con- 
venience, the  meter  is  placed  in  the  cellar  instead  of  the  attic. 


FIG.  154.  —  How  the  electric  current  is  brought  into  the  home. 

Inside  the  building  the  wires  are  joined  in  series  to  cut-out 
fuses,  the  switch,  the  meter,  cut-out  fuses,  and  the  wires  lead- 
ing to  the  electric  lights. 

Fuse  wire.  —  All  electric  house  fixtures  are  protected  from 
damage  from  an  excess  of  current  by  a  device  known  as  a  fuse. 
It  is  simply  a  piece  of  wire  which  melts  when  the  current  is 
too  great.  Fuse  wire  is  made  of  an  alloy  which  melts  at  a  low 
temperature ;  it  is  made  in  different  sizes,  and  the  size  used  in 
any  case  depends  upon  the  current  strength  which  may  be  used 
in  the  appliance.  For  example,  if  the  maximum  current  allow- 
able in  an  electric  device  is  three  amperes,  a  three-ampere  fuse 
is  used  to  protect  it.  If  the  current  goes  above  three  amperes, 
the  fuse  melts  and  the  current  is  stopped.  The  fuse  wire  is 
usually  inclosed  in  a  porcelain  cylinder  or  other  insulating  de- 
vice to  avoid  danger  of  fire  from  the  melted  metal.  In  Fig.  154 


2l6  PHYSICS  OF  THE  HOUSEHOLD 

the  first  cut-out  fuse  protects  all  appliances  in  the  house.  If  it 
is  a  lighting  circuit  and  one  of  these  fuses  is  melted,  all  the  lights 
go  out.  The  fuses  on  the  branch  lines  protect  the  lights  and 
other  appliances  on  that  particular  branch. 

The  cut-out  fuses  and  switch  are  placed  in  a  cabinet  lined  with 
slate,  marble,  or  asbestos,  as  a  further  protection  against  fire. 

RESISTANCE,  SERIES  AND  PARALLEL  CONNECTIONS 

Resistance.  —  All  conductors  of  electricity  offer  some  resist- 
ance to  the  passage  of  the  electric  current.  The  laws  of  this 
resistance  are  as  follows : 

The  resistance  of  any  substance 

(1)  is  directly  proportional  to  its  length ; 

(2)  is  inversely  proportional  to  its  area  of  cross  section ; 

(3)  varies  with  the  nature  of  the  substance ; 

(4)  changes  with  the  temperature. 

To  put  these  laws  into  simple  language : 

If  two  wires  are  the  same,  except  that  one  is  twice  as  long 
as  the  other,  the  longer  wire  will  have  twice  the  resistance  of 
the  shorter  wire. 

If  two  wires  are  the  same  in  all  respects,  except  that  the  area 
of  cross  section  of  one  is  twice  that  of  the  other,  the  resistance 
of  the  thicker  wire  is  one  half  that  of  the  thinner  wire. 

The  resistance  depends  upon  the  material  of  which  the  wire 
is  made ;  for  example,  if  we  compare  the  resistance  of  an  iron 
wire  with  that  of  a  copper  wire  of  exactly  the  same  dimensions, 
we  find  that  the  iron  wire  has  about  six  times  as  much  resist- 
ance as  the  copper  wire. 

The  resistance  varies  with  the  temperature.  The  resistance 
of  all  metals  increases  as  we  increase  the  temperature ;  the 
resistance  of  carbon  and  electrolytes  decreases  as  we  increase 
the  temperature. 

Series  and  parallel  connection.  —  Wires  joined  end  to  end 
in  such  a  way  that  all  of  the  current  passes  through  each  wire 
are  said  to  be  joined  in  series  (Fig.  155).  In  this  case  the  total 


MEASURING  INSTRUMENTS 


217 


resistance  is  equal  to  the  sum  of  the  two  resistances.     In  Fig. 
155,  it  is  equal  to  RI  +  R%. 

Wires  joined,  as  in  Fig.  156,  in  such  a  way  that  only  part  of 
the  current  flows  through  each  wire,  are  said  to  be  joined  in 


A         Ri        o       *i         B 
^-^VV\WW\/VV\-*-AWWAM/W-W 


FIG.  155.  —  Wires  joined  in  series. 


FIG.   156.  —  Wires  joined 
in  parallel. 


parallel.    If  in  this  case  the  resistances  are  equal,  the  resist- 
ance of  the  combination  is  equal  to  one  half  that  of  each  wire. 

If  we  let  R  =  the  total  resistance  and  E  the  e.  m.  f  .  between 

•p 

A  and  B,  the  total  current  C  is  equal  to  —  and  is  equal  to  the 

K 

sum  of  the  currents  in  each  branch  ;  that  is,  C  =  d  -f-  C2  ;  that  is, 


R 


R 


This  is  the  formula  for  finding  the  total  resistance  of  two  wires 
joined  in  parallel. 

Cells  in  series  and  parallel.  —  If  we  join  N  cells  in  series, 
we  find  the  e.  m.  f  .  of  the  combination  to  be  N  times  the  e.  m.  f. 


4v     3v    3w    2v     2v     v 


Cells  in  series.  Cells  in  Parallel. 

FIG.  157.  —  Cells  joined  in  series  and  in  parallel. 

of  each  cell.  In  Fig.  157,  the  e.  m.  f.  of  one  cell  is  V,  of  4  cells 
4  V.  We  find  also  that  the  total  resistance  is  N  times  the  resist- 
ance of  one  cell. 


2l8  PHYSICS  OF  THE  HOUSEHOLD 

If  we  let  C  =  the  current,  E  =  e.  m.  f .  of  one  cell,  r  =  the 
resistance  of  one  cell,  and  R  =  the  external  resistance,  the 
current  is  found  by  the  formula 

C=     N  E 
Nr  +  R 

If  we  join  N  cells  in  parallel,  we  find  the  total  e.  m.  f.  to  be 
equal  to  that  of  one  cell.  In  Fig.  157,  the  e.  m.  f.  of  three  cells 
joined  in  parallel  is  V.  We  find  also  that  the  total  resistance  of 

the  cells  is  —  times  that  of  one  cell. 

N 

We  find  the  current  in  this  case  by  the  formula 

E 


When  the  external  resistance  is  large,  we  join  the  cells  in 
series ;  when  the  external  resistance  is  small,  we  join  them  in 
parallel. 

EXERCISES 

1.  Describe  the  two  types  of  galvanometers. 

2.  Describe  the  voltmeter  and  ammeter. 

3.  What  is  a  fuse  wire? 

4.  State  the  laws  of  resistance. 

5.  Two  wires  are  joined  in  series ;   the  resistance  of  each  is  3  ohms. 
What  is  the  total  resistance? 

6.  What  is  the  total  resistance  if  the  wires  in  5  are  joined  in  parallel  ? 

7.  The  e.  m.  f .  of  a  dry  cell  is  1.5  volts;    the  internal  resistance  is  .1 
ohm.  What  current  will  5  such  cells  joined  in  series  send  through  a  bell 
having  a  resistance  of  7  ohms? 


CHAPTER  XXI 
INDUCED   CURRENTS.     THE   DYNAMO 

Private  lighting  plant.  —  A  private  lighting  plant  for  country 
residences  is  shown  in  Fig.  158.  The  outfit  consists  of  a  gaso- 
line engine,  dynamo,  switchboard,  and  storage  battery.  The 
gasoline  engine  runs  the  dynamo,  the  current  from  the  dynamo 


' 


FIG.  158. — A  private  electric  lighting  plant. 

charges  the  storage  cells,  and  the  current  from  the  storage  cells 
is  used  in  the  lights  as  needed.  The  electric  lights  used  are 
32-volt  tungsten  lamps.  Since  a  storage  cell  has  a  voltage  of 
2  volts,  1 6  storage  cells  are  connected  in  series  to  make  a  32- 
volt  storage  battery. 

We  have  studied  electric  lights,  the  storage  cell,  and  also  the 
gasoline  engine.     We  may  now  study  the  dynamo. 

219 


220  PHYSICS   OF  THE  HOUSEHOLD 

The  dynamo.  —  There  are  two  chief  sources  of  the  electric 
current,  namely,  the  electric  cell  and  the  dynamo.  The 
electric  cell  is  used  only  when  the  electric  power  required  is 
small;  for  example,  to  operate  the  electric  bell.  Where 
the  electric  power  needed  is  comparatively  large,  the  current 
is  obtained  from  a  dynamo ;  for  example,  to  operate  heat- 
ing, cooking,  and  lighting  appliances,  and  electric  motors. 
The  dynamo  is  the  most  important  of  electrical  appliances, 
because  it  furnishes  the  current  for  nearly  all  other  electrical 
devices.  It  is  usually  located  at  the  power  house  of  the 
electric  light  and  power  company,  and  is  driven  by  a  steam 
engine,  gas  engine,  gasoline  engine,  or  by  water  power. 
We  study  it  because  we  wish  to  understand  the  source  of 
the  current  used  in  those  household  appliances  which  use 
more  power  than  can  be  supplied  economically  by  electric 
cells. 

Before  we  take  up  the  study  of  the  dynamo  it  will  be  neces- 
sary to  learn  something  about  induced  currents  because  the 
dynamo  produces  induced  currents. 

Induced  Currents.  —  In  1831,  the  English  scientist,  Michael 
Faraday,  discovered  that  an  electric  current  could  be  produced 
in  a  wire  by  means  of  a  magnet.  He  called  these  currents 
induced  currents. 

We  can  illustrate  the  production  of  induced  currents  by  means 
.of  the  following  experiment,  Fig.  159. 

If  we  connect  the  ends  of  a  coil  of  wire  with  a  galvanometer, 
and  then  plunge  a  magnet  into  the  coil,  we  find  that  a  momen- 
tary current  is  produced  in  the  coil.  If  we  lift  the  magnet  out 
of  the  coil,  a  momentary  current  is  produced  in  the  opposite 
direction.  This  is  a  remarkable  experiment.  We  take  a  coil 
which  has  no  current  in  it,  and  a  magnet  which  also  has  no  cur- 
rent in  it,  and  by  pushing  the  magnet  into  the  coil  we  obtain  a 
momentary  current.  This  is  a  very  important  discovery  be- 
cause it  gives  us  another  method  of  producing  an  electric 
current.  It  is  the  basis  of  the  dynamo ;  and  without  the  dynamo 


INDUCED    CURRENTS.     THE   DYNAMO 


221 


the  greater  part  of  the  modern  electrical  development  would 
have  been  impossible. 


GAUVi 


FIG.  159.  —  Producing  induced  currents. 

Magnetic  lines  of  force  are  cut  by  the  coil.  —  When  the 
magnet  enters  or  leaves  the  coil,  the  magnetic  lines  of  force  of 
the  magnet  are  cut  by  each  turn  of  wire  of  the  coil.  It  is  this 
operation  which  produces  the  current. 

These  experiments  have  been  made  many  times  with  great 
care,  and  it  has  been  learned  that  when  magnetic  lines  of  force 
are  cut  by  any  conductor,  an  electromotive  force  is  set  up  in 


222  PHYSICS  OF  THE  HOUSEHOLD 

the  conductor ;  and  that  the  direction  of  the  electromotive  force 
depends  upon  the  direction  in  which  the  magnetic  lines  of  force 
are  cut.  Let  us  put  this  into  ordinary  language. 

In  our  experiments  with  magnets,  pages  171-172,  we  placed 
a  magnet  under  a  piece  of  paper,  and  by  dusting  iron  filings 
over  the  paper  we  traced  out  the  magnetic  lines  of  force  of  the 
magnet.  Now  copper  wire  is  a  conductor,  and  when  we  push 
a  pole  of  a  magnet  into  a  coil  of  copper  wire,  each  loop  of  the 
coil  cuts  many  lines  of  force  of  the  magnet,  and  an  electro- 
motive force  is  set  up  in  the  coil.  When  we  allow  the  magnet 
to  remain  at  rest,  there  are  no  magnetic  lines  of  force  being  cut, 
and  no  electromotive  force  is  set  up  in  the  coil.  When  the 
magnet  pole  is  pulled  out  of  the  coil,  the  magnetic  lines  of  force 
are  cut  in  the  opposite  direction,  and  an  electromotive  force 
in  the  opposite  direction  is  set  up  in  the  coil. 

Strength  of  e.  m.  f .  in  coil.  —  If  we  push  the  magnet  into 
the  coil  slowly  and  then  rapidly,  we  notice  that  the  e.  m.  f. 
produced  is  much  greater  when  the  motion  is  rapid. 

If  we  hold  the  like  poles  of  two  magnets  together,  and  push 
them  into  the  coil,  the  number  of  magnetic  lines  of  force  cut 
is  twice  as  great,  and  we  find  that  the  e.  m.  f .  set  up  is  twice  as 
great. 

If  we  join  to  the  galvanometer  a  coil  made  of  wire  of  the  same 
size,  but  with  more  turns,  and  push  the  magnet  pole  into  it, 
we  find  the  e.  m.  f.  to  be  greater. 

After  many  experiments  it  has  been  learned  that :  "  the 
strength  of  the  electromotive  force  set  up  in  a  conductor  de- 
pends upon  the  number  of  magnetic  lines  of  force  cut  per  second." 
Or,  in  other  words,  the  strength  of  the  electromotive  force  set 
up  in  a  coil  depends  upon  :  the  strength  of  the  magnet,  the  number 
of  turns  of  wire  in  the  coil,  and  upon  the  speed  with  which  the 
magnetic  lines  of  force  are  cut. 

Direction  of  the  induced  current.  —  In  the  experiments  above 
we  have  used  the  north  pole  of  a  permanent  magnet.  If  we 
use  the  south  pole,  we  find  that  when  the  south  pole  enters  the 


INDUCED   CURRENTS.     THE  DYNAMO  223 

coils,  it  produces  an  induced  current  in  the  opposite  direction  to 
that  of  the  current  produced  when  the  north  pole  enters. 

When  the  south  pole  leaves  the  coil,  it  produces  a  current 
opposite  in  direction  to  that  produced  when  a  north  pole  leaves. 

Similar  results  are  obtained  when  an  electromagnet  is  used 
instead  of  a  permanent  magnet.  In  this  case,  however,  instead 
of  moving  the  electromagnet  into  and  out  of  the  coil,  we  can 
place  it  in  the  coil  and  simply  start  and  stop  the  current. 

If,  in  each  case,  we  find  the  direction  of  the  induced  current 
in  the  coil,  we  can  find  the  direction  of  the  magnetic  field  pro- 
duced by  it  (by  means  of  the  rule  given  on  page  178).  We 
find  that  tlie  magnetic  field  produced  in  the  coil  by  the  induced 
current  is  opposite  in  direction  to  the  magnetic  field  in  the  object 
which  produces  the  induced  current.  This  is  Lenz's  law. 

In  other  words,  when  a  north  pole  enters  a  coil,  the  magnetic 
field  produced  in  the  coil  is  opposite  to  that  in  the  magnet. 
Magnetic  fields  which  are  opposite  in  direction  oppose  each 
other.  Thus  the  magnetic  field  produced  in  the  coil  always 
opposes  the  motion  of  the  object  which  produces  it. 

We  have  learned,  then, 

(1)  that  an  electromotive  force  is  set  up  in  a  conductor  when- 
ever it  cuts  magnetic  lines  of  force ; 

(2)  that  the  strength  of  the  electromotive  force  depends 
upon  the  number  of  magnetic  lines  of  force  cut  per  second ; 

(3)  that  the  direction  of  the  induced  current  is  always  such 
that  its  magnetic  field  opposes  the  motion  of  that  which  pro- 
duces it. 

APPLICATIONS  OF  INDUCED  CURRENTS 

The  dynamo.  —  The  construction  of  the  dynamo  is  based 
upon  the  laws  stated  above.  The  essential  parts  of  a  dynamo 
are  a  coil  of  wire  and  a  magnet,  Fig.  160.  The  coil  of  wire  AB 
is  so  arranged  that  it  can  be  revolved  on  the  axis  CD  between 
the  poles  of  the  magnet.  If  we  examine  Fig.  160,  the  magnetic 
lines  of  force,  represented  by  the  long  arrows,  extend  from  the 


224 


PHYSICS  OF  THE  HOUSEHOLD 


north  pole  to  the  south  pole  of  the  magnet.     Every  time  the 
coil  makes  one  revolution,  each  side  of  the  coil  cuts  the  magnetic 

lines  of  force  twice, 
first  in  one  direction 
and  then  in  the  other. 
There  are  thus  two 
currents  produced  in 
the  coil  at  each  revo- 
lution: the  first  is 
set  up  during  the 
first  half  revolution, 
and  is  in  one  direc- 


FIG.  1 60.  —  Principle  of  the  dynamo. 


tion ;    the  second  is 
set    up    during    the 

second  half  revolution,  and  is  in  the  opposite  direction.     Thus 

if  the  coil  makes  five  revolutions  each  second,  there  are  ten 

separate  currents  set   up 

each  second,  five  in  one 

direction,  and  five  in  the 

other.     A   current   which 

is  reversed  in  direction  a 

number  of  times  a  second 

is    called    an    alternating 

current. 
The  coil  which  revolves 

between  the  poles  of  the 

magnet  is  called  the  arma- 
ture of  the  dynamo.     It  is 

always  wound  on  a  soft 

iron  core  because  the  mag- 
netic lines  of  force  pass 

through  iron  more  readily 

than    through    air,    and 

therefore  the  number  of  lines  of  force  passing  through  the  coil  is 

increased.     The  magnet  is  called  the  field  magnet  of  the  dynamo. 


FIG.  161.  —  Diagram  of  an  alternating- 
current  dynamo. 


INDUCED   CURRENTS.     THE   DYNAMO 


225 


The  alternating-current  dynamo.  —  The  current  is  generated 
in  the  armature,  and  in  order  to  make  use  of  it,  means  must 
be  provided  to  lead  it  out  of  the  armature  into  an  outer 
circuit.  The  chief  difference  between  the  alternating-current 
dynamo  and  direct-current  dynamo  is  in  the  way  in  which  this 
is  done. 

A  simple  diagram  of  the  alternating-current  dynamo  is  given 
in  Fig.  161.  The  current  is  led  out  of  the  armature  AB  as  fol- 
lows: One  end  of  the  wire  of  the  armature  is  joined  to  the 
metal  ring  5,  the  other  to  the  metal  ring  Sf.  These  rings  are 
fastened  to  the  axle  CD,  and  revolve  with  it ;  they  are  insulated 
from  each  other  and  from  the  axle.  Two  metal  or  carbon  strips 
E  and  F  are  so  arranged  that  one  touches  one  ring  and  the  other 
the  other.  These  strips  are  called  brushes;  they  are  stationary 
and  the  rings  slide  under  them.  The  current  is  led  out  of  the 
armature  and  into  the  outer  circuit  L  through  these  brushes. 

The  current  sent  through  the  lights  L  is  alternating  because 
it  comes  out  on  one  brush  . 

during  one  half   revolu-  *^H  W 

tion,  and  on  the  other 
during  the  other  half  rev- 
olution. A  dynamo  ar- 
ranged in  this  way  is 
called  an  alternating-cur- 
rent dynamo. 

The  direct-current 
dynamo.  —  The  direct- 
current  dynamo  is  the 
same  as  the  alternating- 
current  dynamo,  except 
that  it  has  a  commutator 
instead  of  rings.  The 
commutator  is  a  ring 
split  in  sections  as  shown 
in  Fig.  162.  One  end  of 
Q 


FIG.  162.  —  Diagram  of  a  direct-current 
dynamo. 


226 


PHYSICS  OF  THE  HOUSEHOLD 


the  coil  of  the  armature  is  joined  to  one  section  C  and  the 
other  to  the  other  section  C.  During  one  half  revolution  of 
the  armature  the  current  conies  out  on  one  section,  and  during 
the  other  half  on  the  other ;  but  since  the  commutator  revolves 
with  the  armature,  the  current  always  comes  out  into  the  outer 
circuit  from  one  brush  and  returns  on  the  other.  Thus  the 
current  in  the  outer  circuit  always  flows  in  the  same  direction, 
that  is,  it  is  a  direct  current.  Dynamos  arranged  in  this  way 
are  called  direct-current  dynamos. 

Dynamos  and  motor.  —  We  can  show  that  the  dynamo  and 
motor  are  the  same  in  construction,  as  follows.     Join  two  hand- 


GENERATOR  MOTOR 

FIG.   163.  —  The  dynamo  or  generator  and  the  motor. 

power  dynamos  by  means  of  wires,  Fig.  163.  Then  turn  the 
armature  of  the  first  dynamo  by  hand  power  and  the  current 
produced  makes  the  armature  of  the  second  dynamo  revolve. 
That  is,  the  second  dynamo  acts  as  a  motor.  Now  turn  the 
armature  of  the  second  dynamo  by  hand  power  and  the  current 
produced  makes  the  armature  of  the  first  dynamo  revolve. 
That  is,  the  first  dynamo  acts  as  a  motor. 

This  shows  that  the  dynamo  and  motor  are  the  same  in  con- 
struction. A  machine  is  acting  as  a  dynamo  when  it  turns 
mechanical  energy  into  electrical  energy ;  it  is  acting  as  a  motor 
when  it  turns  electrical  energy  into  mechanical  energy. 

Armature  and  field  magnets.  —  The  armatures  of  commercial 
dynamos  consist  of  many  separate  coils  wound  on  a  soft  iron 


INDUCED   CURRENTS.     THE  DYNAMO 


227 


core.  If  the  core  is  in  the  shape  of  a  drum,  the  armature  is 
called  a  drum  armature.  If  the  core  is  in  the  shape  of  a  ring,  the 
armature  is  called  a  ring  armature. 

The  commutators  of  direct-current  dynamos  have  as  many 
sections  as  there  are  coils  on  the  armature. 

The  field  magnet  of  a  direct-current  dynamo  is  energized  by 
part  of  the  current  produced  by  the  dynamo.  The  field  magnet 
of  an  alternating-current  dynamo  is  energized  by  a  small  sepa- 
rate direct-current  dynamo. 

Commercial  dynamos.  —  A  commercial  direct-current  dynamo 
is  shown  in  Fig.  164.  It  will  be  noticed  that  the  field  magnet 


FIG.  164. —  A  commercial  direct-current  dynamo. 

Y  has  six  poles.  These  are  alternately  north  and  south 
poles.  The  armature  B  B,  shown  on  the  right,  is  a  drum 
armature.  It  consists  of  many  coils  wound  lengthwise  on  a 
soft  iron  drum.  The  coils  are  attached  to  the  strips  of  the 
commutator  in  series. 

A  commercial  alternating-current  dynamo  is  shown  in  Fig..i65. 
The  field  magnet  has  many  separate  poles.  The  armature  is 
of  the  ring  type.  There  is  no  commutator,  the  alternating 
current  being  led  from  the  armature  by  means  of  rings  on  the 
armature  axle. 


228 


PHYSICS   OF  THE  HOUSEHOLD 


FIG.   165. — A  commercial  alternating-current  dynamo. 

The  sources  of  the  electrical  energy.  —  The  electricity  which 
we  use  in  our  homes  comes  from  the  power  house  of  an  elec- 
tric power  company,  except  that  which  we  obtain  from  batteries. 
If  the  electric  power  company  drive  their  dynamos  by  means  of 
a  steam  engine,  the  following  changes  in  energy  take  place. 
The  coal  burned  under  the  boiler  produces  steam  in  the  boiler, 
the  steam  drives  the  steam  engine,  and  the  steam  engine  drives 
the  dynamo.  The  heat  energy  of  coal  is  thus  changed  to  me- 
chanical energy  in  the  boiler  and  engine ;  the  mechanical  energy 
is  changed  into  electrical  energy  in  the  dynamo ;  and  the  elec- 
trical energy  is  turned  into  light  and  heat  energy  in  the  appli- 
ance. We  see  then  that  the  electrical  energy  which  we  use  in 
household  appliances  is  in  this  case  derived  from  the  heat  energy 
of  the  coal  burned  under  the  boiler. 

In  some  power  houses  the  dynamos  are  driven  by  a  gas  en- 
gine or  a  gasoline  engine.  In  this  case  the  electrical  energy  is 
obtained  from  the  heat  energy  of  the  gas  or  gasoline  which  burns 
in  the  engine. 

In  other  power  houses  the  dynamos  are  driven  by  waterwheels. 
In  this  case  the  electrical  energy  is  obtained  from  the  mechanical 
energy  of  the  falling  water  which  passes  through  the  waterwheel. 


INDUCED   CURRENTS.     THE  DYNAMO  229 

From  the  above  we  see  that  when  we  use  an  electrical  appli- 
ance in  our  homes,  a  certain  extra  amount  of  coal,  gas,  gasoline, 
or  water  must  be  used  in  the  power  house  to  supply  the  neces- 
sary current.  For  example,  when  the  current  comes  from  a 
power  house  where  coal  is  used,  and  we  use  an  electric  stove, 
iron,  toaster,  or  motor  in  our  homes,  a  certain  extra  amount  of 
coal  must  be  burned  to  supply  the  current.  We  see  also  that 
if  we  leave  an  electric  light  or  other  appliance  "  on  "  longer  than 
it  is  needed,  we  really  waste  a  certain  amount  of  coal  in  the 
power  house. 

EXERCISES 

1.  Why  is  the  dynamo  an  important  electrical  appliance? 

2.  Describe  how  induced  currents  are  produced. 

3.  On  what  does  the  e.  m.  f.  of  an  induced  current  depend? 

4.  State  Lenz's  law. 

5.  Make  a  simple  diagram  of  an  alternating-current  dynamo. 

6.  Make  a  simple  diagram  of  a  direct-current  dynamo. 

7.  How  would  you  show  that  a  dynamo  and  motor  are  the  same  in 
construction  ? 

8.  Name  the  changes  in  energy  which  occur  when  coal  is  used  to 
produce  electricity  for  lighting. 


CHAPTER  XXII 
INDUCED    CURRENTS    (continued) 

TRANSFORMER,  INDUCTION  COIL,  AND  TELEPHONE 

The  transformer.  —  In  cities  and  towns,  the  current  supplied 
for  lighting  purposes  is  alternating.  A  high  voltage  current  is 
generated  in  an  alternating-current  dynamo  at  the  power  house. 
It  is  carried  on  wires  to  a  transformer  near  the  group  of  houses 
in  which  it  is  to  be  used.  In  the  transformer  it  produces  a  new 
alternating  current  of  lower  voltage.  This  new  current  is  then 

distributed  to  the  houses.  We 
will  first  study  the  transformer, 
and  then  take  up  its  advantages. 
A  diagram  of  the  transformer  is 
given  in  Fig.  166.  It  consists  of 
a  ring  of  soft  iron  upon  which  are 

FIG.  166.  -The  transformer.          WOUnd    the    tw°    Coils    P    and    S' 

The   coil    P,  which  receives  the 

current  from  the  dynamo,  is  called  the  primary  coil.  The 
coil  S,  in  which  the  new  current  is  produced,  is  called  the 
secondary  coil. 

The  action  of  the  transformer  is  as  follows.  When  the  al- 
ternating current  from  the  dynamo  passes  through  the  wire 
of  the  coil  P  in  one  direction,  it  makes  the  ring  a  magnet. 
This  magnetism  passes  through  the  secondary  coil  S  and  pro- 
duces an  induced  current.  The  next  instant  the  direction  of 
the  current  in  P  changes ;  this  magnetizes  the  ring  in  the  op- 
posite direction,  and  produces  an  induced  current  in  the  opposite 
direction  in  the  coil  S.  If,  for  example,  the  current  from  the 

230 


INDUCED   CURRENTS  231 

dynamo  reverses  its  direction  50  times  a  second,  the  induced 
current  produced  in  5  also  reverses  50  times  a  second. 

The  important  feature  of  the  transformer  is  that  it  takes  an 
alternating  current  of  one  voltage  and  produces  a  new  alter- 
nating current  of  a  higher  or  lower  voltage  according  to  the 
ratio  of  the  windings  of  the  coils.  For  example,  if  the  coil  P 
(Fig.  1 66)  has  ten  times  as  many  turns  of  wire  as  the  coil  5, 
the  new  current  produced  in  S  has  only  TV  the  voltage  of  the 
current  used  in  P.  If,  however,  the  current  from  the  dynamo 
is  sent  into  the  coil  S,  the  new  induced  current  obtained  from  P 
has  10  times  the  voltage  of  the  current  used  in  5.  If  the  wind- 
ings are  in  any  other  ratio  a  corresponding  change  is  produced. 

In  general,  the  ratio  of  the  voltages  of  the  old  and  new  currents 
is  the  same  as  the  ratio  of  the  number  of  turns  of  wire  in  the  coils. 

When  a  transformer  is  used  to  take  an  alternating  current  of 
one  voltage,  and  produce  a  new  alternating  current  of  a  lower 
voltage,  it  is  called  a  step-down  transformer.  When  it  is  used  to 
produce  a  current  of  a  higher  voltage,  it  is  called  a  step-up 
transformer. 

The  power  of  a  current  is  measured  in  watts,  and  is  equal  to 
volts  X  amperes.  This  power  is  not  altered  by  a  transformer. 
For  example,  if  the  ratio  of  the  windings  of  the  coils  is  10,  a 
current  of  noo  volts  and  20  amperes  is  transformed  to  one  of 
no  volts  and  200  amperes.  That  is,  the  amperage  is  changed 
in  the  opposite  direction  to  the  voltage,  and  the  product,  volts  X 
amperes,  is  constant,  noo  X  20  =  no  X  200.  This  is  true 
in  general,  if  we  neglect  the  small  loss  in  the  transformer. 

Advantage  of  the  transformer.  —  In  city  lighting  circuits  the 
current  from  the  dynamo  has  usually  a  potential  of  noo  volts. 
This  is  carried  to  the  transformer  where  it  produces  a  new  cur- 
rent of  no  volts.  The  advantage  of  this  arrangement  is  that 
the  high  voltage  current  can  be  carried  on  smaller  wires  because 
the  amperage  is  lower.  The  size  of  wire  needed  for  any  cur- 
rent depends  only  upon  the  number  of  amperes.  For  example, 
the  size  of  wire  needed  to  carry  20  amperes  is  only  one  tenth 


PHYSICS  OF  THE  HOUSEHOLD 


that  required  for  200  amperes.  The  wire  is  made  of  copper, 
and  since  copper  is  expensive,  the  saving  in  cost  of  copper, 
brought  about  by  the  use  of  the  transformer,  is  a  very  important 
consideration. 

The  induction  coil.  —  We  have  now  studied  two  appliances 
which  produce  induced  currents ;  namely,  the  dynamo  and  the 
transformer.  In  this  paragraph  we  will  take  up  another  such 
appliance,  the  induction  coil. 

The  induction  coil  is  applied  in  a  number  of  ways ;  for  example, 
it  is  used  as  a  jump  spark  coil  on  gasoline  engines  in  automobiles, 

motor  boats,  etc.  It 
is  used  also  to  produce 
current  for  the  X  ray 
and  wireless  telegraph, 
which  we  will  study 
later.  It  is  also  an 
important  part  of  the 
telephone. 

•A  diagram  of  the 
induction  coil  is  shown 
in  Fig.  167.  It  con- 
sists of  a  soft  iron  core  C  on  which  is  wound  a  primary  coil 
PP'  consisting  of  a  few  turns  of  coarse  wire.  Outside  of  this 
is  wound  a  secondary  coil  SS' ,  consisting  of  many  turns  of  fine 
wire.  The  primary  wire  is  connected  with  a  battery  and  an 
interrupter,  which  consists  of  a  piece  of  soft  iron  a  on  the  end 
of  a  piece  of  spring  steel  5.  The  current  from  the  battery  B 
passes  through  the  primary  coil  PP',  and  to  the  steel  spring  s ; 
it  then  passes  across  to  the  large  screw  at  the  point  b,  and  back 
-to  the  battery  through  the  metal  post  O.  The  interrupter 
"  makes  "  and  "  breaks  "  the  current  at  the  point  b.  When 
the  current  is  started,  C  becomes  a  magnet  and  draws  a  over, 
breaking  the  current  at  b.  The  core  C  then  ceases  to  be  a  mag- 
net and  the  spring  s  draws  a  back ;  this  "  makes  "  the  current; 
and  so  on. 


FIG.  167.  —  The  induction  coil. 


INDUCED   CURRENTS  233 

The  action  of  the  induction  coil  is  as  follows:  When  the 
current  is  started  in  the  primary,  an  induced  current  is  pro- 
duced in  the  secondary,  in  a  direction  opposite  to  that  in  the 
primary.  When  the  current  is  stopped  in  the  primary,  an  in- 
duced current  is  produced  in  the  secondary,  in  the  same  direc- 
tion as  that  in  the  primary.  Since  the  secondary  has  many 
more  turns  of  wire  than  the  primary,  the  induced  current  in  the 
secondary  has  a  much  higher  e.  m.  f .  than  that  in  the  primary. 
If  the  e.  m.  f .  is  high  enough,  sparks  pass  between  the  secondary 
terminals. 

The  condenser  C,  shown  at  the  bottom  of  the  diagram,  serves 
to  make  the  "  break  "  sharper.  It  consists  of  sheets  of  metal 
foil  insulated  from  one  another  by  paraffined  paper.  Its  action 
is  as  follows:  We  know  from  our  study  of  induced  currents 
that  the  e.  m.  f .  produced  is  greater  the  more  rapidly  the  mag- 
netic lines  of  force  are  cut.  If  the  condenser  is  not  used,  the 
current  in  the  primary  forms  a  small  arc  at  b  on  the  "  break,'' 
making  the  "  break  "  slow.  This  arc  is  due  to  the  current 
produced  by  induction  in  the  primary  coil.  When  a  condenser 
is  used,  the  current  flows  into  the  condenser  and  the  arc  is  not 
formed.  Thus  the  "  break  "  is  sharp  and  the  e.  m.  f.  produced 
in  the  secondary  coil  is  high. 

The  telephone.  —  The  telephone,  as  we  all  know,  is  a  very 
important  household  electrical  appliance.  The  chief  parts  of 
it  are  the  transmitter  and  receiver.  The  part  of  the  telephone 
into  which  we  speak  is  the  transmitter,  and  the  part  we  place 
against  the  ear  is  the  receiver. 

The  transmitter.  —  Behind  the  mouth  piece  of  the  transmitter, 
Fig.  1 68,  is  a  thin,  flexible  sheet  metal  diaphragm  D,  fastened 
loosely  around  the  edge  in  such  a  way  that  it  vibrates  when 
sound  waves  strike  it.  A  small  plate  of  carbon  C  is  fastened 
to  the  back  of  this  diaphragm,  and  vibrates  with  it.  A  second 
plate  of  carbon  C'  is  fastened  to  the  transmitter  frame  a  short 
distance  behind  this,  and  the  space  between  the  two  carbon 
plates  is  filled  with  carbon  granules  or  carbon  shot  g.  The 


234 


PHYSICS  OF  THE  HOUSEHOLD 


current  from  the  battery  flows  in  succession  through  the  dia- 
phragm, the  first  carbon  plate,  the  carbon  shot,  and  the  second 
carbon  plate.  It  then  flows  through 
the  primary  of  induction  coil,  and  back 
to  the  battery. 

The  action  of  the  transmitter,  illus- 
trated in  Fig.  169,  is  as  follows  :  When 
a  person  speaks  into  the  mouthpiece  T, 
the  sound  waves,  produced  in  the  air, 
make  the  thin,  flexible  diaphragm  vibrate, 
and  with  it  the  carbon  plate  fastened  at 
the  back.  When  the  plate  moves  in, 
the  carbon  granules  are  forced  more  closely  together;  and 
when  it  moves  out,  they  are  less  closely  compressed.  When 
carbon  granules  are  forced  together,  they  conduct  electricity 
better  than  they  do  when  less  closely  compressed.  Therefore, 
when  the  diaphragm  moves  in,  the  battery  B  sends  a  stronger 


FIG.  1 68.  —  The  trans- 
mitter. 


FIG.  169.  —  Diagram  of  two  telephones  connected. 

current  through  the  primary  coil  P  of  the  induction  coil ;  and 
when  it  moves  out,  this  current  is  decreased.  When  the 
current  is  increasing  in  the  primary  coil,  an  induced  current 


INDUCED   CURRENTS 


235 


is  produced  in  one  direction  in  the  secondary  S  of  the  induction 
coil ;  and  when  it  is  decreasing,  another  induced  current  is  pro- 
duced in  the  secondary  but  in  the  opposite  direction  to  the  first. 

If  the  voice  produces  200  sound  waves  per  second,  the  thin, 
flexible  diaphragm  moves  in  and  out  200  times  each  second, 
and  thus  there  are  400  induced  currents  produced  in  the  second- 
ary of  the  induction  coil  each  second,  200  in  each  direction. 
These  induced  currents  pass  through  the  receiver  R  of  the 
telephone  spoken  into,  and  also  through  the  receiver  Rf  of  the 
telephone  at  which  the  message  is  received.  Let  us  see  what 
happens  in  the  receiver. 

The  receiver.  —  If  we  unscrew  the  cap  of  the  receiver  of  a 
telephone,  we  find  a  thin,  flexible  sheet  iron  plate  D,  and  beneath 
it  a  magnet  A  A,  Fig.  170.  The  plate  is  sup- 
ported around  the  edge  in  such  a  way  that  it  is 
close  to  the  pole  of  the  magnet,  but  does  not 
touch  it.  The  magnet  is  a  permanent  magnet 
with  a  coil  of  fine  wire  CC  wound  around  one 
pole.  The  two  end  wires  FF  of  the  coil  are 
joined  to  the  secondary  of  the  induction  coil. 
The  magnet  then  is  really  a  permanent  magnet 
and  also  an  electromagnet.  In  modern  receivers, 
a  horseshoe  magnet  is  used  and  both  poles  are 
placed  beneath  the  plate  D. 

The  action  of  the  receiver  is  as  follows  :  The 
thin,  flexible  plate  is  bent  in  by  the  pull  of  the 
permanent  magnet,  and  when  an  induced  current  passes  around 
the  coil  in  such  a  direction  that  it  increases  the  strength  of  the 
permanent  magnet  pole,  the  flexible  plate  is  bent  in  still  farther. 
The  next  induced  current  being  in  the  opposite  direction  de- 
creases the  strength  of  the  permanent  magnet  pole,  and  allows 
the  elasticity  of  the  iron  to  bend  the  plate  out.  The  next 
current  bends  it  in,  the  next  out,  etc.  If  the  voice  produces 
200  sound  waves  in  the  transmitter,  the  induced  currents  bend 
the  flexible  plate  of  the  receiver  in  and  out  200  times  each 


FIG.   170.  — The 
receiver. 


236  PHYSICS  OF  THE  HOUSEHOLD 

second.  The  plate  thus  produces  200  sound  waves  per  second 
in  the  air  in  the  cap  of  the  receiver,  and  the  ear  -hears  a  sound 
which  is  made  up  of  200  waves. 

We  notice,  then,  that  the  telephone  wire  carries  not  sound, 
but  electricity.  When  a  person  speaks  into  a  telephone 
the  sound  moves  just  about  one  inch,  that  is,  to  the  flexible 
plate  of  the  transmitter.  After  this  it  is  all  electric  currents 
until  we  come  to  the  receiver.  The  sound  a  person  hears  in 
the  receiver  is  produced  about  one  inch  from  the  ear ;  it  is  pro- 
duced by  the  movement  of  the  flexible  plate  of  the  receiver. 

EXERCISES 

1.  Make  a  diagram  of  the  transformer.      Describe  what  takes  place 
in  the  transformer. 

2.  Make  a  diagram  of  an  induction  coil.     Describe  it. 

3.  Make  a  diagram  of  the  telephone  transmitter,  and  tell  what  takes 
place  when  a  person  speaks  into  it. 

4.  Make  a  diagram  of  a  telephone  receiver,  describe  it,  and  tell  how 
sound  is  produced  in  it. 

5.  Make  a  diagram  of  two  telephones  connected.     Describe  it. 


CHAPTER  XXIII 


WIRELESS  TELEGRAPH,  CATHODE  RAYS,  X  RAYS,  AND 

RADIUM 


Wireless  telegraphy. 

-  The  telegraph  and 
telephone  transmit 
messages  from  one 
place  to  another  by 
means  of  wires.  The 
wireless  telegraph,  as 
its  name  indicates, 
transmits  messages 
without  the  use  of 
wires  between  sta- 
tions. The  essential 
parts  of  the  wireless 
telegraph  are  the 
sending  apparatus 
and  the  receiving  ap- 
paratus. 

Wireless  sending  ap- 
paratus. —  A  simple 
sending  station  is 
shown  in  Fig.  171. 
It  consists  of  an  in- 
duction coil  7  with 
one  secondary  termi- 
nal connected  to  the 
earth  and  the  other 


-r  Earth 


FIG.  171.  —  Wireless  sending  apparatus. 
237 


PHYSICS  OF  THE  HOUSEHOLD 


connected  with  a  group 
of  wires  suspended  from 
insulators  attached  to  a 
tall  mast.  This  group 
of  wires  is  called  the 
aerial. 

An  electric  spark 
which  appears  to  the 
eye  to  be  one  single 
spark  is  really  made  up 
of  from  10  to  30  sepa- 
rate sparks.  This  may 
be  shown  by  looking  at 
the  image  of  a  spark 
in  a  rapidly  revolving 
mirror,  when  the  image 
appears  as  a  series  of 
sparks  side  by  side, 
ft  may  be  illustrated 
more  simply  by  allow- 
ing an  electric  spark  to 
penetrate  a  piece  of 
cardboard;  the  rim  of 
the  hole  will  be  rough 
on  both  sides,  showing 
that  the  electricity 
must  have  passed 
through  it  in  both 
directions. 

When  a  spark  is  pro- 
.  duced  at  the  spark  gap 

,7  by  the   induction   coil 

ric.  172. — Wireless  receiving  apparatus. 

of  the  sending  station, 

the  electricity  passes  back  and  forth  between  the  aerial  and 
the  earth  a  number  of  times.    This  surging  of  electricity  up 


WIRELESS   TELEGRAPH  239 

and  down  produces  a  series  of  waves  in  the  ether.  These 
waves  travel  out  in  all  directions,  and  are  detected  at  the 
receiving  station  as  explained  below. 

Wireless  receiving  station.  —  A  simple  receiving  station  is 
shown  in  Fig.  172.  It  is  equipped  with  an  aerial  similar  to 
that  at  the  sending  station.  This  is  joined  to  one  end  of  the 
coherer,  which  is  the  sensitive  part  of  the  receiving  station. 
The  other  end  of  the  coherer  is  connected  with  the  ground. 

The  coherer.  —  The  action  of  the  coherer  is  based  upon  the 
following  facts.  Metal  filings  conduct  electricity  very  poorly; 
but  when  an  electric  spark  is  passed  through  them,  the  filings 
cling  together  or  cohere,  and  become  a  good  conductor  of  elec- 
tricity. If  the  filings  are  shaken  up,  they  separate  or  decohere 
and  again  become  a  poor  conductor  of  electricity. 

A, ,   B 


C 

FIG.  173.  —  The  coherer. 

The  coherer,  Fig.  173,  consists  of  a  small  glass  tube  in  which 
two  metal  rods  A ,  B  are  separated  by  a  small  quantity  of  loosely 
packed  metal  filings  C. 

The  coherer,  besides  being  connected  with  the  aerial  and  the 
ground  wire,  is  also  connected  in  series  with  the  battery  B,  and 
the  relay  magnet  M.  When  the  waves  from  the  sending 
station  fall  upon  the  aerial,  electrical  surgings  are  set  up  between 
the  aerial  and  the  ground  and  a  series  of  sparks  pass  through 
the  metal  filings  in  the  coherer.  The  filings  cohere  and  become 
a  conductor,  a  current  then  flows  from  the  battery  B,  through 
the  coherer,  and  through  the  relay  magnet  M . 

The  bell  battery  b  is  connected  with  the  electric  bell  through 
the  relay  armature  A.  When  the  current  from  the  battery  B 
flows  through  the  relay  magnet,  the  relay  armature  is  drawn 
over.  This  closes  the  circuit  of  the  bell  battery  b,  at  the  point 
p,  and  the  bell  begins  to  ring. 


240  PHYSICS  OF  THE  HOUSEHOLD 

The  knob  of  the  bell  is  so  arranged  that  it  strikes  the  coherer 
and  shakes  up  the  filings.  This  decoheres  the  filings,  and  they 
become  a  poor  conductor,  therefore  the  current  through  the  re- 
lay magnet  is  stopped,  the  relay  armature  is  released,  the  cur- 
rent from  the  bell  battery  is  broken  at  the  point  p,  and  the  bell 
stops  ringing.  As  long  as  the  waves  are  coming  from  the  send- 
ing station  the  bell  rings  because  the  filings  remain  cohered  in 
spite  of  the  fact  that  the  bell  knob  is  shaking  them  up.  When 
the  waves  stop  coming  the  last  stroke  of  the  bell  knob  decoheres 
the  filings,  and  the  bell  stops  ringing. 

In  sending  a  message  the  operator  at  the  sending  station 
sends  out  long  and  short  series  of  waves.  These  produce  long 
and  short  rings  of  the  bell,  which  correspond  to  the  dash  and 
dot  of  the  telegraph  code.  An  ordinary  telegraph  sounder  may 
be  used  instead  of  the  bell,  and,  with  coherers  which  do  not 
require  tapping  to  decohere  them,  the  telephone  receiver  may 
be  used.  Each  station  is  equipped  with  both  a  sending  and  a 
receiving  apparatus,  which  use  the  same  aerial,  one  being  dis- 
connected while  the  other  is  in  use. 

Cathode  rays,  electrons.  —  An  electric  discharge  passes 
through  a  partial  vacuum  much  more  readily  than  through  air. 

This  may  be  shown 
as  follows.  Connect 
the  terminal  of  an 
induction  coil  with 
FIG.  174-  — Electricity  passing  through  a  the  two  wires,  sealed 

partial  vacuum.  . 

into   the   ends   of    a 

glass  tube,  Fig.  174,  attach  an  air  pump  to  the  small  open 
tube  shown  near  one  end.  Start  the  induction  coil  working  and 
the  spark  passes  between  the  terminals  of  the  induction  coil; 
but  when  the  air  pressure  inside  the  tube  is  reduced  to  about 
-^V  of  an  atmosphere,  the  spark  passes  through  the  long  tube, 
in  the  form  of  a  colored  band.  That  'is,  an  electric  discharge 
passes  more  readily  through  a  partial  vacuum  than  through  air. 
When  the  pressure  is  reduced  to  about  -^TOT  of  an  atmos- 


WIRELESS  TELEGRAPH  241 

sphere  the  nature  of  the  discharge  changes,  and  invisible  radia- 
tions known  as  cathode  rays  are  given  off  from  the  cathode. 
The  properties  of  these  cathode  rays  are  as  follows : 

(1)  They  produce  fluorescence  in  the  glass  walls  of  the  tube. 

(2)  When  concentrated  on  a  substance,  they  produce  strong 
heating  effects. 

(3)  They  make  sharp  shadows  of  objects  placed  in  their  path. 

(4)  They  give  a  negative  charge  of  electricity  to  bodies  they 
fall  upon. 

(5)  They  are  deflected  by  a  magnet. 

(6)  They  are  deflected  by  an  electric  charge. 

(7)  When  they  penetrate  a  gas  they  make  the  gas  a  con- 
ductor of  electricity.     This  is  called  ionization. 

Nature  of  cathode  rays.  —  The  cathode  rays  have  been  the 
subject  of  much  investigation,  and  it  has  been  found  that  they 
consist  of  streams  of  very  small  particles  which  are  called  elec- 
trons. The  electrons  are  isolated  charges  of  negative  electricity 
which  travel  with  various  velocities  up  to  \  that  of  light.  Ab*/ 
the  two  remarkable  facts  have  been  discovered:  first,  that  all 
electrons  are  the  same,  no  matter  what  metal  is  used  as  the  cath- 
ode, or  what  gas  remains  in  the  tube ;  second,  that  the  mass  of 
the  electron  is  electromagnetic  in  nature  and  thus  depends  on 
the  velocity,  but  in  certain  cases  is  about  yyVrr  the  mass  of  the 
hydrogen  atom.  In  other  words,  it  has  been  discovered  that 
there  are  particles  very  much  lighter  than  the  lightest  atom 
known,  and  that  these  particles  are  the  same,  no  matter  from 
what  substance  they  are  derived. 

Electron  theory  of  matter.  —  The  discoveries  mentioned  in 
the  last  paragraph  gave  rise  to  the  electron  theory  of  matter 
developed  by  the  English  physicist,  Sir.  J.  J.  Thomson.  This 
theory  is  that  all  atoms  are  made  up  of  systems  of  positively 
and  negatively  charged  particles  in  rapid  motion,  the  negatively 
charged  particles  being  electrons.  According  to  this  theory: 
light,  heat,  and  electromagnetic  waves  are  produced  by  the  vibra- 
tion of  the  electrons ;  a  current  of  electricity  consists  of  a  stream 


242 


PHYSICS  OF  THE  HOUSEHOLD 


of  electrons  moving  through  the  conductor ;  a  negatively  charged 
body  contains  an  excess  of  electrons ;  and  a  positively  charged 

body  is  deficient  in 
electrons. 

X  rays.  —  When 
the  cathode  rays 
fall  upon  a  body 
such  as  a  piece  of 
metal  or  the  walls 
of  a  tube,  they  set 
up  another  in  visible 
radiation  known  as 
This  was  discovered  by  Rontgen  in  1895.  The 


FIG.   175.  —  X  ray  tube.     The  cathode  rays  from  K 
strike  the  metal  anode  A  and  produce  X  rays. 


the  X  ray. 


X  ray  is  different  from 
deflected  by  a  magnet  or 
by  an  electric  charge; 
and  it  penetrates  sub- 
stances through  which 
the  cathode  ray  cannot 
pass. 

X  ray  pictures. — The 
X  ray  is  similar  to  light 
in  that  it  affects  a  photo- 
graphic plate  and  causes 
fluorescence  in  certain 
substances.  The  denser 
a  substance  the  less 
readily  the  X  rays  pene- 
trate it.  If  the  hand  is 
held  over  a  photographic 
plate  inclosed  in  a  plate 
holder,  and  is  exposed 
to  the  influence  of  the 
X  ray  for  a  short  time, 
the  plate  on  being  de- 


the  cathode  rays  in  that  it  is  not 


FIG.   176.  —  X  ray  photograph. 


WIRELESS  TELEGRAPH  243 

veloped  shows  the  outline  of  the  hand  and  of  the  bones  of  the 
hand.  The  bones  are  denser  than  the  flesh,  and  the  X  rays 
penetrate  them  less  readily ;  therefore  the  outline  of  the  bones 
on  the  negative  is  lighter  than  that  of  the  flesh.  A  picture 
printed  from  this  negative  shows  the  bones  darker  Fig.  176. 

Similarly,  if  the  hand  is  held  between  an  X  ray  tube  and  a 
screen  covered  with  a  substance  which  is  caused  to  fluoresce 
by  X  rays,  the  outline  of  the  hand  and  of  the 
bones  of  the  hand  may  be  seen  on  the  screen. 
The  fluoroscope,  Fig.  177,  is  used  for  this  pur- 
pose. The  bottom  of  it  is  a  fluorescent  screen. 

Nature  of  X  rays.  —  X  rays  were  called  X  rays 
because  it  was  not  known  what  they  are.  It  has 
recently  been  shown  that  they  are  similar  to  light 
in  that  they  can  be  reflected,  refracted,  and  polar-  FlG-  177-— The 

•      j       mi  IT          i         i  i  •  i         •  fluoroscope. 

ized.     1  hey  are  believed  to  be  very  thin  pulses  in 

the  ether,  set  up  by  the  impact  of  electrons  upon  solid  bodies. 

Radium.  —  X  rays  are  produced  by  cathode  rays  which  cause 
strong  fluorescent  and  phosphorescent  effects  in  various  bodies. 
This  led  scientists  to  examine  bodies  which  are  made  phos- 
phorescent by  sunlight  to  see  whether  they  gave  off  X  rays. 
In  1896  the  French  scientist,  Henri  Becquerel,  investigated  this 
with  the  phosphorescent  substance  uranium.  He  wrapped  a 
photographic  plate  in  opaque  paper,  placed  a  coin  on  the  paper, 
and  suspended  a  piece  of  uranium  over  the  coin  and  plate. 
He  allowed  these  to  stand  for  a  few  days  in  a  dark  room,  and  on 
developing  the  plate  found  upon  it  an  image  of  the  coin  similar 
to  that  formed  by  X  rays.  It  has  since  been  shown  that  there 
is  no  relation  between  phosphorescence  and  this  effect;  but 
Becquerel's  experiment  demonstrated  that  uranium  sends  out 
rays  which  have  an  effect  similar  to  those  produced  by  X  rays. 

Professor  and  Madame  Curie  then  investigated  many  sub- 
stances and  found  that  the  element  thorium  has  a  similar  ef- 
fect. During  the  investigation  they  discovered  that  pitch- 
blende, an  ore  of  uranium,  has  a  greater  effect  than  uranium. 


244  PHYSICS  OF  THE  HOUSEHOLD 

They  concluded  that  there  must  be  in  pitchblende  some  new 
substance  producing  this  greater  effect.  They  started  in  to 
separate  this  new  substance,  and  after  great  labor  succeeded 
in  separating  a  few  milligrams  of  it  from  tons  of  pitchblende. 
This  new  substance  they  called  radium. 

It  has  been  shown  by  Rutherford  that  the  rays  given  off 
by  radium  are  of  three  kinds,  which  he  named  the  alpha,  beta, 
and  gamma  rays.  The  alpha  rays  are  positively  charged  par- 
ticles with  a  mass  equal  to  four  times  that  of  the  hydrogen  atom 
and  with  a  velocity  of  about  20,000  miles  per  second.  It  has 
recently  been  shown  that  they  are  positively  charged  atoms  of 
helium.  The  beta  rays  are  identical  with  cathode  rays ;  they 
consist  of  streams  of  electrons  with  very  high  velocities.  The 
gamma  rays  are  supposed  to  be  X  rays  produced  by  the  impact 
of  the  electrons  of  the  beta  rays  on  surrounding  substances. 

Substances  which  have  the  property  of  sending  out  rays  are 
known  as  radioactive  substances;  such  are  uranium,  thorium, 
radium,  actinium,  and  polonium. 

It  has  been  shown  that  the  atoms  of  radioactive  substances, 
besides  giving  off  rays,  also  decompose  and  produce  atoms  of 
new  substances.  For  example,  uranium  atoms  decompose, 
and  after  the  formation  of  two  intermediate  substances  pro- 
duce atoms  of  radium.  Radium  atoms  in  turn  produce  a  series 
of  new  atoms,  the  final  one  at  present  known  being  lead. 
Similarly,  thorium  and  actinium  atoms  decompose  and  each 
produce  a  series  of  new  atoms. 

Radioactive  substances  are  intensely  interesting  because, 
first,  their  atoms  change  into  atoms  of  new  substances ;  second, 
the  atoms  in  changing  give  off  energy  in  the  form  of  heat  and  of 
particles  moving  with  great  velocity.  This  energy  is  the  in- 
ternal energy  of  the  atoms. 

In  radioactive  substances  we  have  for  the  first  time  examples 
of  the  change  of  one  element  into  another,  and  of  the  release  of 
the  energy  contained  in  the  atom. 


WIRELESS  TELEGRAPH  245 

EXERCISES 

1.  Make  a  diagram  of  a  wireless  sending  apparatus  and  describe  it. 

2.  Make  a  diagram  of  a  wireless  receiving  apparatus  and  describe  it. 

3.  State  the  properties  of  cathode  rays. 

4.  What  are  the  properties  of  electrons? 

5.  State  the  electron  theory  of  matter. 

6.  How  are  X  rays  produced.     How  are  X  ray  photographs  made? 

7.  What  are  the  alpha,  beta,  and  gamma  rays? 

8.  Why  are  radioactive  substances  interesting  ? 


CHAPTER  XXIV 
LIGHT   IN   THE   HOME 

How  do  we  see  ?  —  We  see  any  object  by  means  of  light 
which  travels  from  the  object  to  the  eye.  Light  which  enters  a 
room  from  any  source  is  reflected  many  times  from  the  floor, 
ceiling,  walls,  and  from  objects  in  the  room.  That  is,  it  is 
scattered  or  diffused  in  all  directions  and  thus  objects  in  the 
room  are  made  visible.  Any  particular  object,  however,  is  seen 
only  by  means  of  the  light  which  moves  from  that  object  to  the 
eye. 

Arrangement  of  lighting  fixtures  in  the  home.  —  The  lighting 
fixtures  in  a  home  should  be  so  arranged  that  the  light  is  thrown 
upon  the  objects  to  be  seen,  and  not  into  the  eyes.  To  under- 
stand the  reason  for  this  we  must  understand  the  action  of  the 
pupil  of  the  eye.  The  pupil  is  the  opening  in  the  colored  part 
of  the  eye.  Its  function  is  to  regulate  the  amount  of  light 
which  enters  the  eye.  When  the  light  is  strong,  the  pupil  con- 
tracts ;  when  the  light  is  dim,  it  expands. 

It  is  difficult  to  read  while  facing  a  strong  light  because  the 
strong  light  causes  the  pupil  to  contract  to  such  an  extent  that 
it  does  not  admit  sufficient  light  from  the  book  or  paper.  It 
is  easy  to  read  with  the  back  to  the  light,  because  the  strong 
light  does  not  fall  upon  the  eye,  and  the  pupil  expands  until  it 
admits  sufficient  light  from  the  book  or  paper. 

In  planning  the  arrangement  of  the  lighting  fixtures  we  must 
remember :  first,  that  an  object  is  seen  by  means  of  light  which 
travels  from  the  object  to  the  eye ;  therefore,  the  fixtures  should 
be  so  placed  that  they  throw  the  light  on  the  objects  we  wish  to 

246 


LIGHT  IN  THE  HOME  247 

see ;  second,  the  pupil  adjusts  itself  to  the  light  which  falls  on 
it ;  therefore,  the  fixtures  should  be  so  arranged  that  the  direct 
light  does  not  fall  on  the  eyes.  In  short,  the  fixtures  should  be 
so  arranged  that  the  light  is  thrown  on  the  objects  to  be  seen 
and  not  into  the  eyes. 

Keeping  these  facts  in  mind,  we  find  the  proper  illumination 
for  the  different  rooms  in  the  home  to  be  somewhat  as  follows. 
In  the  kitchen,  separate  lamps  over  the  table,  range,  and  sink, 
each  lamp  being  so  shaded  that  the  light  is  thrown  down  upon 
the  table,  range,  or  sink  and  not  into  the  eyes.  If  in  addition, 
general  illumination  is  desired,  a  separate  light  should  be  placed 
near  the  ceiling,  well  above  the  level  of  the  eyes.  Similarly 
in  the  dining  room,  it  would  seem  that  the  lights  should  be  well 
shaded  and  placed  low  over  the  table,  in  order  to  throw  the 
light  down  upon  the  table  and  not  into  the  eyes.  In  the  halls, 
where  general  illumination  is  desired,  the  lights  should  be  near 
the  ceiling  at  a  height  well  above  the  level  of  the  eye.  In  the 
dressing  rooms,  lights  should  be  placed  on  each  side  of  the  mirror 
and  a  sufficient  distance  in  front  of  the  mirror  to  throw  the  light 
on  the  sides  of  the  face  rather  than  into  the  eyes, 

Intensity  of  light.  —  If  we  hold  a  book  one  foot  from  a  lamp, 
it  receives  a  certain  amount  of  light;  if  we  hold  it  two  feet 
from  the  lamp,  it  receives  only  one  fourth  as  much  light;  if 
we  hold  it  three  feet  from  the  lamp,  it  receives  only  one  ninth 
as  much  light ;  and  so  on.  That  is,  the  intensity  of  the  light 
on  an  object  varies  inversely  as  the  square  of  the  distance  between 
the  object  and  the  source  of  light. 

We  can  show  this  nicely  by  means  of  the  experiment  illus- 
trated in  Fig.  178.  The  light  from  a  lamp  is  allowed  to  pass 
through  a  small  hole  O,  and  fall  upon  a  square  hole  A  BCD  in 
a  screen  one  foot  from  O.  The  light  which  passes  through  the 
square  hole  is  caught  on  a  second  screen  placed  two  feet  from  O. 
It  covers  an  area  E  F  G  H  which  has  four  times  the  area  of  the 
square  hole.  Since  the  light  covers  four  times  the  area,  it  is 
only  one  fourth  as  intense  on  any  one  area.  That  is,  the  light 


248  PHYSICS  OF  THE  HOUSEHOLD 

at  a  distance  of  two  feet  is  only  one  fourth  as  intense  as  it  is 
at  a  distance  of  one  foot  from  the  source. 

If  the  second  screen  is  placed  three  feet  from  O,  the  area 
covered  is  nine  times  as  great,  that  is,  the  intensity  is  only  one 


FlG.  178.  —  The  intensity  of  light  varies  inversely  as  the  square  of  the  distance 
between  the  source  of  light  and  the  object  illuminated. 

ninth  as  great.  That  is,  the  intensity  of  the  light  on  an  object 
varies  inversely  as  the  square  of  the  distance  between  the  object 
and  the  light. 

Candle  power  of  a  light.  —  In  the  days  of  our  great-grand- 
parents, the  common  source  of  light  in  the  home  was  the  candle. 
Since  that  time  the  oil  lamp  has  been  perfected,  and  methods  of 
lighting  by  means  of  gas  and  electricity  have  been  introduced. 
With  the  introduction  of  each  new  source  of  light,  the  amount 
of  light  it  gave  was  compared  to  the  amount  given  by  a  candle. 
If,  for  example,  the  light  gave  five  times  as  much  light  as  a  candle, 
it  was  called  a  five  candle-power  light.  This  is  the  common 
method  still  in  use;  for  example,  if  we  examine  the  common 
electric  light  bulb,  we  find  it  marked  16  c.  p.,  which  stands  for 
1 6  candle  power  and  means  that  the  electric  light  gives  as  much 
light  as  1 6  standard  candles  or  16  times  as  much  light  as  one 
standard  candle. 

The  law  stated  above  regarding  the  intensity  of  light  is  used 
in  finding  the  candle  power  of  a  lamp.  The  apparatus  used  in 
measuring  candle  power  is  called  a  photometer.  We  shall 
study  one  of  the  simplest  of  these,  namely,  Bunsen's  pho- 
tometer. 

Bunsen's  photometer.  —  The  arrangement  of  Bunsen's  photo- 
meter is  shown  in  Fig.  179.  A  sheet  of  paper  A  with  a  greased 
spot  in  the  center  is  placed  between  a  standard  candle  B  and  the 


LIGHT   IN  THE   HOME 


249 


lamp  to  be  tested,  C.  The  standard  candle  is  placed  a  certain 
distance  from  the  screen,  say  one  foot.  The  lamp  is  then  moved 
back  and  forth  until  a  position  is  found  in  which  the  greased 
spot  is  as  bright  as  the  remainder  of  the  paper.  If  this  distance 
is  3  ft.,  the  lamp  at  3  ft.  is  throwing  as  much  light  on  the  screen 
as  the  candle  at  i  ft.  Therefore,  since  the  intensity  of  the  light 
is  inversely  proportional  to  the  square  of  the  distance,  the  lamp 
is  9  candle  power. 

Let  us  explain  this  further.     If  a  piece  of  paper  with  a  greased 
spot  is  viewed  by  reflected  light,   the  greased  spot  appears 


FIG.  179.  —  Bunsen's  photometer. 

darker  than  the  remainder  of  the  paper,  because  more  light 
passes  through  the  greased  spot  than  through  the  paper.  If  it 
is  viewed  by  transmitted  light,  the  greased  spot  is  brighter  than 
the  paper,  for  the  same  reason,  that  is,  more  light  passes  through 
the  greased  spot  than  through. the  paper. 

The  candle  throws  a  certain  amount  of  light  on  the  screen 
of  the  photometer,  and  a  certain  amount  passes  through  the 
greased  spot.  When  the  lamp  is  in  such  a  position  that  it  throws 
the  same  amount  of  light  on  the  screen  as  the  candle,  the  greased 
spot  is  as  bright  as  the  remainder  of  the  paper,  because  the  light 


250  PHYSICS  OF  THE  HOUSEHOLD 

which  passes  through  from  one  side  is  equal  to  that  which  passes 
through  from  the  other  side. 

If  the  candle  is  placed  i  ft.  from  the  screen  and  the  lamp  throws 
an  equal  amount  of  light  on  the  screen  when  it  is  3  ft.  from  the 
screen,  it  is  a  9  candle-power  lamp,  because  the  intensity  varies 
inversely  as  the  square  of  the  distance,  and  the  lamp  at  i  ft. 
from  the  screen  would  throw  on  the  screen  9  times  as  much 
light  as  the  candle.  If  other  lamps  at  4,  5,  and  10  ft.  throw  as 
much  light  on  the  screen  as  the  candle  at  i  ft.,  they  are  16,  25, 
and  100  candle-power  lamps  respectively. 

Nature  of  light.  —  Light  is  that  which  produces  the  sensation 
of  sight.  The  exact  nature  of  light  has  long  been  a  subject 
of  investigation  and  at  the  present  time  scientists  believe  that 
light  is  a  transverse  wave  motion  in  the  ether.  The  ether  is  be- 
lieved to  be  a  medium  which  fills  all  space.  It  is  believed  to  fill 
the  space  between  the  planets  and  also  the  space  between  the 
molecules  and  atoms  of  all  substances.  For  example,  it  is  be- 
lieved to  fill  the  space  between  the  sun  and  the  earth  and 
also  the  space  between  the  molecules  and  atoms  of  our 
bodies.  It  is  believed  that  when  any  body  is  moved,  the 
ether  flows  through  the  body  just  as  water  flows  through  a 
sieve. 

A  transverse  wave  motion  is  one  in  which  the  particles  of  the 
medium  vibrate  in  a  direction  at  right  angles  to  the  direction 
in  which  the  wave  moves.  For  example,  water  waves  are  trans- 
verse waves ;  the  wave  moves  along  the  surface  of  the  water, 
but  any  particle  of  water  moves  simply  up  and  down  in  a  direc- 
tion at  right  angles  to  the  surface.  This  can  be  illustrated  by 
throwing  a  chip  on  the  surface  of  water  on  which  there  are 
waves  which  do  not  break  or  form  white  caps.  The  wave  moves 
along  the  surface,  but,  if  there  is  no  wind,  the  chip  moves  simply 
up  and  down  at  right  angles  to  the  surface. 

Light  waves  are  heat  waves.  —  If  the  hand  is  held  near  a 
source  of  light,  such  as  the  flame  of  a  candle,  oil  lamp,  or  gas 
jet,  the  hand  is  illuminated  and  also  heated.  All  our  common 


LIGHT  IN  THE   HOME  251 

sources  of  light  send  out  thousands  of  streams  of  waves  in  the 
ether.  Each  stream  consists  of  waves  of  one  length,  but  the 
waves  in  one  stream  may  differ  in  length  from  those  of  another. 
Therefore  at  any  instant  the  hand  is  receiving  waves  of  many 
different  wave  lengths.  All  the  waves  received  give  some  heat ; 
that  is,  they  are  all  heat  waves;  a  certain  number  also  produce 
upon  our  eyes  the  sensation  of  sight,  that  is,  they  are  light  as 
well  as  heat  waves.  Light  waves,  then,  are  heat  waves  which 
produce  upon  our  eyes  the  sensation  of  sight. 

The  process  of  transferring  heat  from  one  place  to  another 
by  means  of  heat  waves  is  known  as  radiation.  We  took  up 
the  question  of  radiation  on  page  104  above;  we  also  learned 
there  how  heat  and  light  waves  are  produced. 

How  heat  and  light  waves  are  produced.  —  In  the  section  on 
heat  we  learned  that  when  a  body  is  heated  the  particles  com- 
prising the  body  vibrate  more  rapidly.  All  our  sources  of  light 
are  heated  bodies,  for  example,  the  sun,  candle,  oil  lamp,  gas  jet, 
incandescent  light,  arc  light,  etc.  At  the  present  time  scientists 
believe  that  heat  and  light  waves  are  produced  as  follows.  The 
molecules,  atoms,  and  particularly  the  very  small  particles, 
known  as  electrons,  of  which  the  atoms  are  composed,  vibrate 
rapidly  in  the  luminous  part  of  the  source  of  light.  Each 
vibrating  particle  sets  up  a  stream  of  waves  in  the  ether.  These 
waves  travel  out  in  all  directions  and  are  the  heat  and  light 
waves. 

Velocity  of  heat  and  light  waves.  —  The  velocity  of  heat  and 
light  waves  has  been  measured  many  times,  and  it  has  been 
found :  first,  that  all  heat  and  light  waves  travel  hi  the  ether 
with  the  same  velocity,  and  second,  that  they  travel  with  the 
enormous  velocity  of  186,000  miles  per  second.  It  is  very  hard 
to  conceive  such  a  velocity.  Sound  travels  at  a  velocity  of 
about  £  mile  per  second,  a  high  velocity  bullet  at  the  rate  of 
about  J  mile  per  second.  These  are  the  highest  velocities  with 
which  we  are  familiar,  but  they  are  snail's  paces  compared  to  the 
velocity  of  light. 


252  PHYSICS  OF  THE  HOUSEHOLD 

Wave  front,  ray,  pencil,  and  beam.  —  Every  point  of  a  lumi- 
nous body  sends  out  waves  in  all  directions  in  the  ether.  These 
waves  travel  with  equal  velocities.  Each  vibration  of  a  particle 
sets  up  a  wave  which  travels  out  from  the  particle  in  the  form 
of  a  sphere  of  which  the  particle  is  the  center.  The  surface 
of  this  sphere  is  called  the  wave  front. 

Lines  drawn  from  the  vibrating  particle  through  the  wave 
fronts  are  called  rays.  They  show  simply  the  directions  in 
which  the  light  is  traveling.  Since  the  vibrating  particle  is 
the  center  from  which  the  wave  fronts  start,  a  ray  is  a  radius 
of  the  sphere  and  therefore  a  ray  .is  always  perpendicular  to  the 
wave  fronts. 

When  rays  meet  at  a  point  or  diverge  from  a  point,  they  are 
known  as  a  pencil  of  light. 

When  the  source  of  light  is  at  a  great  distance,  the  wave 
fronts  are  nearly  parallel,  and  as  a  result  the  rays  are  nearly 
'parallel.  A  collection  of  parallel  rays  is  called  a  beam  of  light. 

.EXERCISES 

1.  How  would  you  show  that  objects  are  seen  by  light  which  travels 
from  the  object  to  the  eye  ? 

2.  Why  is  it  more  comfortable  to  read  with  the  back  to  the  source  of 
light? 

3.  How  should  the  lights  be  placed  in  a  kitchen,  dining  room,  hall, 
and  dressing  rooms?     Why? 

4.  Describe  an  experiment  to  show  that  the  intensity  of  illumination 
varies  inversely  as  the  square  of  the  distance  between  the  source  of 
light  and  the  object  illuminated. 

5.  Describe  the  Bunsen  photometer. 

6.  Lamps  placed  3!,  5,  and  10  ft.  from  a  Bunsen  photometer  screen 
give  the  same  light  as  a  standard  candle  placed  i  foot  from  the  screen. 
What  is  the  candle  power  of  the  lamp  in  each  case? 

7.  What  is  light  ? 

8.  How  do  light  waves  differ  from  other  heat  waves? 

9.  Define  wave  front,  ray,  pencil,  and  beam. 


CHAPTER  XXV 


FIG.  1 80.  —  Light  travels  in  a  straight  line. 


REFLECTION   AND    REFRACTION    OF   LIGHT 

Light  travels  in  straight  lines  in  a  given  medium.  —  That 
through  which  light  travels  is  known  as  the  medium  through 
which  it  travels ;  examples,  ether,  air,  water,  and  glass. 

Light  travels  in  a  straight 
line  through  a  given  medi- 
um. This  can  be  shown 
for  the  medium,  air,  by 
the  experiment  illustrated 
in  Fig.  1 80.  A  hole  is 
made  in  each  of  three 
cards,  and  the  cards  are 
placed  in  a  row  in  front  of  a  candle.  The  light  of  the  candle 
can  be  seen  only  when  the  holes  are  in  a  straight  line. 

When  light  moving  through  one  medium  meets  another  medium, 
it  is  reflected  or  refracted  or  both.     For  example,  when  light  mov- 
p  ing   through   air   strikes   a   mirror,    it    is 

reflected  ;    when    it    strikes   water,   it    is 
partly  reflected  and  partly  refracted. 

Angle  of  reflection  equals  angle  of  in- 
cidence.— If  a  beam  of  sunlight  is  allowed 
to  fall  upon  a  mirror  and  the  beam  before 
and  after  reflection  is  made  visible  by  means 
of  chalk  dust  in  the  air,  it  is  found  that  the  beams  make  equal 
angles  with  a  ruler  held  perpendicular  to  the  mirror,  Fig.  181. 

The  beam  7  which  strikes  the  mirror  is  called  the  incident 
beam  and  the  beam  R  which  is  reflected  is  called  the  reflected 

253 


FIG.  181.  — The  angle 
of  reflection  is  equal  to 
the  angle  of  incidence. 


254  PHYSICS  OF  THE  HOUSEHOLD 

beam.  The  angle  i  which  the  incident  beam  makes  with  the 
perpendicular  PN  is  called  the  angle  of  incidence,  and  the  angle 
r  which  the  reflected  beam  makes  with  the  perpendicular  is 
called  the  angle  of  reflection.  The  experiment  described  above 
illustrates  the  Law  of  Reflection,  which  is,  The  angle  of  reflec- 
tion equals  the  angle  of  incidence. 

The  location  of  the  image  seen  in  a  mirror.  —  The  image  seen 
in  a  mirror  is  always  the  same  distance  behind  the  mirror  that  the 
object  is  in  front;  also,  the  image  is  exactly  opposite  the  object, 
that  is,  a  line  drawn  from  the  object  perpendicular  to  the  mirror 
passes  through  the  image. 

This  can  be  shown  as  follows  :  Stand  a  pane  of  window  glass 
in  a  perpendicular  position  on  the  table,  place  a  lighted  candle 
in  front  of  it,  and  an  unlighted  candle  of  the  same  size  behind  it. 
Move  the  unlighted  candle  until  it  coincides  with  the  image  of 
the  lighted  candle,  no  matter  from  what  position  the  eye  views 
it  from  the  front  side  of  the  glass.  It  is  found  by  measurement 
that  the  image  is  opposite  the  candle,  and  at  an  equal  distance 
from  the  mirror. 

Since  each  point  of  an  image  is  directly  opposite  the  corre- 
sponding point  of  the  object,  the  image  formed  in  a  mirror  is 
reversed.  For  example,  in  a  mirror,  the  right  hand  becomes  the 
left  hand  and  the  left  hand  the  right  hand. 

The  reason  the  image  appears  the  same  distance  behind  the 
mirror  that  the  object  is  in  front  is  as  follows  :  The  eye  esti- 
mates distance  by  the  curvature  of  the  wave  fronts  which  fall 
upon  it.  If  an  object  is  near  at  hand,  the  wave  fronts  from  it 
are  much  curved ;  if  the  object  is  at  a  distance,  the  wave  fronts 
from  it  are  less  curved  when  they  reach  the  eye. 

The  wave  fronts  from  an  object  which  strike  a  mirror  are 
changed  in  direction  but  not  in  curvature ;  thus  the  image 
appears  to  be  behind  the  mirror  and  at  the  same  distance  be- 
hind that  the  object  is  in  front. 

Refraction.  —  When  light  from  any  object  passes  in  a  slanting 
direction  from  one  medium  to  another,  for  example,  from  water 


REFLECTION  AND   REFRACTION  OF  LIGHT  255 

to  air  or  the  reverse,  the  rays  are  bent  at  the  surface 
separating  the  media.  The  bending  of  the  rays  of  light  is 
called  refraction. 

We  can  show  the  refraction  of  light  rays  when  light  passes 
from  air  into  water  by  the  experiment  illustrated  in  Fig.  182. 
A  beam  of  light  AB  is  allowed  to  pass  through  a  shutter  and 
fall  upon  the  water  in  a  glass  trough.  The  light  after  it  enters 
the  water  moves  in  the  direction  BD  instead  of  in  the  direction 
BC.  It  will  be  noticed  that  the  light  is  bent  towards  the  line 


FIG.  182.  — The  beam  of  light  is  bent  in  passing  from  air  into  water. 

NNf  drawn  perpendicular  to  the  surface  of  the  water  at  the 
point  the  light  enters. 

When  light  passes  from  water  to  air,  the  rays  are  bent  in  the 
opposite  direction,  that  is,  away  from  the  line  drawn  perpen- 
dicular to  the  surface  at  the  point  the  light  leaves  the  water. 

Laws  of  refraction.  —  The  following  laws  of  refraction  have 
been  discovered  by  experiment. 

(i)  When  light  rays  pass  in  a  slanting  direction  from  a  rare 
medium  to  one  more  dense,  they  are  bent  towards  the  line  drawn 
perpendicular  to  the  surface  at  the  point  they  enter. 


256 


PHYSICS   OF   THE   HOUSEHOLD 


(2)  When  light  rays  pass 
in  a  slanting  direction  from 
a  dense  medium  to  one  less 
dense,  they  are  bent  away 
from  the  line  drawn  perpen- 
dicular to  the  surface  at  the 
point  they  leave  the  surface. 
These  laws  are  illustrated 
in  Figs.  183  and  184.  In 
Fig.  183  the  ray  TOR  passes 
from  air  into  water  and  is 
bent  towards  the  line  MOM ' 
drawn  perpendicular  to  the 
surface  at  the  point  it  enters. 

In  Fig.  184  the  ray  10  R  passes  from  water  to  air  and  is  bent 
away  from  the  line  drawn  perpendicular  to  the  surface  at  the 
point  it  leaves  the  water. 

For  any  two  media  the  ratio  between  the  sines  of  the  angles  of  in- 
cidence and  refraction  is  constant.     In  Fig.  183, 


FIG.   183.  —  Light  passing  from  air 
into  water. 


IM  _  NO       .          _'  RM'  _  N'O 

___  ;«»,-.—  . 


and  sine  i  =  NO   =  4 
RO       l     sine  r       N'O       3 


This  ratio  is  constant  for  all  angles  of  incidence  when  light  passes 
from  air  to  water. 
In  Fig.  184, 

sine  i  =  NO  =  3 

sine  r       N'O  ~  4 

This  ratio  is  constant  for  all  angles  of  incidence  when  light  passes  from 
water  to  air. 

Explanation  of  refraction.  —  When  light  passes  from  one 
medium  to  another  of  different  density,  the  curvature  of  the 
wave  fronts  is  altered,  because  the  light  travels  at  a  different 
velocity  in  the  second  medium.  Since  the  rays  are  always  at 
right  angles  to  the  wave  fronts,  their  direction  is  changed 
when  the  wave  fronts  change  in  curvature.  This  is  the  ex- 
planation of  refraction. 


REFLECTION  AND    REFRACTION   OF  LIGHT 


257 


FIG.  184.  —  Light  passing  from  water 
into  air. 


We  may  illustrate  this  by  an  example.     It  is  known  that 

light  travels  faster  in  air  than  in  water,  and  when  we  view  an 

object  in  water,  the  light  moves  from  the  object  to  the  eye, 

first    through   water   and 

then  through  air.    When  a 

wave    front    reaches    the 

surface,   that   part   which 

arrives  first  moves  faster  in 

the  air  than  the  part  which 

is  still  in  water,  thus  the 

curvature    of    the    wave 

fronts  is  increased  and  the 

object  appears  nearer  than 

it  really  is.     Thus  the  rays 

appear  to  come  from  the 

nearer    point,    and    each 

slanting  ray  is  bent  away 

from  the  line  drawn  perpendicular  to  the  surface  at  the  point 

it  leaves  the  surface. 
When  light  travels  from  air  to  water  the  reverse  is  true ;  the 

wave  fronts  become  less  curved,  and  each  slanting  ray  is  bent 

towards  the  perpendicular. 

Media  with  parallel  sides.  —  When  an  object  is  viewed  in 

a  slanting  direction  through  a  dense  medium  with  parallel 
sides,  it  appears  shifted  to  one  side.  The 
explanation  is  illustrated  in  Fig.  185.  Light 
from  A  passes  through  a  glass  plate  with 
parallel  sides.  As  the  light  enters  the  glass 
it  is  bent  towards  the  perpendicular  eg. 
It  travels  in  this  new  direction  through 
the  glass,  and  when  it  leaves  it  is  bent 
away  from  the  new  perpendicular  Jh.  The 

FIG.  185.  —  Light  pass-    ray  after  it  leaves  the  glass  moves  in  a  path 
parallel   Para^el  to  the  path  in  which  it  moved  when 


sides. 


it  entered  the  glass. 


s 


258  PHYSICS  OF  THE  HOUSEHOLD 

Prism.  —  When  an  object  is  seen  through  a  prism  made  of  a 
dense  medium  it  appears  to  be  in  an  entirely  different  position. 
For  example,  in  Fig.  186,  the  object  L  when  seen  through  a 
glass  prism  appears  to  be  at  L'.  The  explanation  is  as  follows : 
When  the  light  from  L  enters  the  glass  prism,  it  passes  from  a 

rare  to  a  dense  medium,  and 

L**^. 

is  bent  towards  the  perpen- 
dicular n.  It  travels  in  this 
new  direction  through  the 
prism.  When  it  leaves  the 

prism,  it  passes  from  a  dense 
FIG.  186.  -  Lpassing  through         to  a  ^  medimn>  and  ig  bent 

away  from  the  perpendicular  n'. 
Since  the  glass  is  in  the  form  of  a  prism,  these  two  bendings  are 
in  the  same  direction  and  the  object  appears  to  be  in  a  position 
much  removed  from  its  actual  position.  It  will  be  noticed 
that  when  light  passes  through  a  prism  it  is  bent  towards  the 
thicker  part  of  the  prism. 

EXERCISES 

1.  State  the  law  of  reflection. 

2.  How  would  you  show  by  experiment  that  an  object  and  its  image 
are  the  same  distance  from  a  plane  mirror? 

3.  Explain  why  the  image  of  an  object  is  the  same  distance  behind  a 
mirror  that  the  object  is  in  front. 

4.  Explain  why  a  glass  of  water  appears  shallower  than  it  is  when 
viewed  from  above. 

5.  State  the  laws  of  refraction. 

6.  Make  a  drawing  showing  the  path  of  a  ray  of  light  through  water 
in  a  bottle  with  flat  parallel  sides ;   explain  each  change  in  direction  of 
the  ray. 

7.  Make  a  drawing  showing  the  path  of  a  ray  of  light  through  a 
prism.     Explain  each  change  in  direction  of  the  ray. 


CHAPTER  XXVI 


__A  conyex  lens 


LENSES   AND    OPTICAL   INSTRUMENTS 

Convex  and  concave  lenses.  —  Lenses  are  of  two  kinds, 
convex  and  concave.  Convex  lenses  are  thicker  in  the  center 
than  at  the  edges.  They  cause  the  light  to  converge.  Concave 
lenses  are  thinner  at  the  center  than  at 
the  edges.  They  cause  the  light  to 
diverge. 

To  understand  the  action  of  lenses,  we 
can  think  of  them  as  made  up  of  sections 
of  prisms,  the  angles  of  the  prisms  being 
greater  the  nearer  we  approach  the  edge. 

In  Fig.  187  a  convex  lens  is  shown  built  up  in  this  way.  A 
ray  of  light  falling  on  one  of  the  prism  sections  is  bent  towards 
the  thicker  part  of  the  section.  If  the  prism  angles  are  properly 
chosen,  the  rays  converge  at  some  point  F ;  and  if  the  incident 
rays  are  parallel,  this  point  F  is  the 
principal  focus  of  the  lens. 

In  Fig.  1 88  is  shown  a  concave  lens 
made  up  of  sections  of  prisms.  In  this 
case  the  thinner  ends  of  the  sections 
point  towards  the  center.  A  ray  of 
light  falling  on  one  of  these  sections  is 
bent  towards  the  thicker  end  of  the 
section.  Thus  the  rays  diverge;  and 

if  the  incident  rays  are  parallel,  the  point  F  from  which 
they  appear  to  come  is  the  principal  focus  of  the  concave 
lens. 

259 


FIG.  188.  —  A  concave  lens, 


260  PHYSICS  OF  THE  HOUSEHOLD 

Images.  —  Images  are  of  two  kinds,  "  real  "  and  "  virtual." 
An  image  is  "  real  "  when  the  rays  which  produce  it  actually 
pass  through  the  image.  An  image  is  "  virtual  "  when  the  rays 
which  produce  it  only  appear  to  pass  through  the  image.  A  real 
image  can  be  produced  on  a  screen ;  a  virtual  image  cannot  be 
produced  on  a  screen. 

Principal  axis,  principal  focus,  focal  length.  —  A  line  drawn 
through  the  centers  of  curvature  of  the  surfaces  of  a  lens  is 
called  the  principal  axis  of  the  lens.  The  point  at  which  rays 
parallel  to  the  principal  axis  converge  (or  from  which  they  ap- 
pear to  diverge)  is  called  the  principal  focus  of  the  lens.  The 
distance  from  the  center  of  the  lens  to  the  principal  focus  is 
called  the  focal  length  of  the  lens. 

Images  formed  by  a  convex  lens.  —  A  convex  lens  produces 
either  real  or  virtual  images.  A  concave  lens  produces  only 
virtual  images. 

When  an  object  is  placed  in  front  of  a  convex  lens  at  a 
distance  greater  than  the  focal  length,  the  lens  produces  a  real 


FIG.   189.  —  Real  image  formed  by  a  convex  lens. 

image.  This  is  illustrated  in  Fig.  189.  The  line  OI  is  the  prin- 
cipal axis.  F  and  F'  are  the  principal  foci.  The  object  is  OB  and 
the  real  image  is  IM .  All  the  rays  from  B  which  fall  on  the  lens 
converge  at  M.  Thus  M  is  the  image  of  the  point  B.  The  rays 
which  fall  on  the  lens  from  a  point  just  below  B  converge  at  a 
point  just  above  M .  Similarly  rays  from  all  points  of  OB  con- 
verge at  corresponding  points  along  IM  and  form  the  real  image. 


I.I.XSES  AND   OPTICAL  INSTRUMENTS 


261 


We  can  trace  readily  the  paths  of  two  or  three  rays  from 
any  point,  and  two  suffice  to  locate  the  image.  We  can  trace 
the  ray  BR,  which  is  parallel  to  the  principal  axis,  because  we 
know  that  it  passes  through  the  focus  F  after  it  leaves  the  lens. 
The  ray  BP  passes  through  the  center  of  the  lens,  and  since 
the  surfaces  of  the  lens  are  nearly  parallel  at  this  point,  the  ray 
is  shifted  a  little  to  one  side.  If  the  lens  is  thin,  however,  we 
make  only  a  slight  error  if  we  assume  that  the  ray  passes  through 
in  a  straight  line.  These  two  rays  locate  the  position  of  M,  the 
image  of  B\  similarly 
other  points  on  IM  are 
found  to  be  images  of 
points  on  OB.  The 
image  IM  is  real  and 
inverted. 

If  an  object  is  placed 
in  front  of  a  convex  lens, 
but  at  a  distance  less 
than  the  focal  length,  a 
virtual  image  is  formed. 
This  is  illustrated  in  Fig. 
190.  The  parallel  ray 
BR  from  the  point  B 

passes  through  the  focus  F  after  leaving  the  lens.  The  ray 
BP  passes  through  the  center  of  the  lens  in  a  straight  line. 
These  rays  are  diverging  after  they  leave  the  lens;  thus  they 
cannot  meet  to  form  a  real  image  of  B.  If,  however,  they  fall 
on  the  eye  of  an  observer,  they  appear  to  come  from  M. 
Thus  M  is  the  virtual  image  of  the  point  B,  and  the  points 
along  IM  are  virtual  images  of  points  along  OB.  The  image 
IM  is  virtual,  erect,  and  enlarged. 

Images  formed  by  a  concave  lens.  —  A  concave  lens  produces 
reduced  virtual  images.  This  is  illustrated  in  Fig.  191.  OB 
is  the  object  and  IM  the  image.  When  the  parallel  ray  BR 
leaves  the  lens,  it  moves  in  such  a  direction  that  it  appears  to 


FiG.   igo.  —  Virtual  image  formed  by  a 
convex  lens. 


262 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  191.  —  Virtual  image  formed  by 
a  concave  lens. 


come  from  the  principal  focus 
F.  The  ray  BP  passes  through 
the  center  of  the  lens  in  a 
straight  line.  These  rays  di- 
verge after  passing  through 
the  lens  and  thus  cannot  form 
a  real  image.  If,  however, 
they  fall  on  the  eye  of  an  ob- 
server they  appear  to  come 
from  M.  Thus  M  is  the  vir- 
tual image  of  B.  Similarly 

IM  is  the  virtual  image  of  OB.    The  image  is  virtual,  erect, 

and  reduced. 

OPTICAL  INSTRUMENTS 

The  camera.  —  The  photographic  camera,  Fig.  192,  is  simply 
a  light-proof  box,  with  a  converging  lens  L,  fixed  in  an  opening 
on  one  side  and  a  ground  glass  screen  S  at  the  opposite  side. 
To  focus  the  camera  on  any  object  AB,  the  lens  is  moved 
back  and  forth  until  a  distinct  inverted  image  ba  is  seen  on 
the  ground  glass  screen.  In  order  to  take  a  picture,  the  lens  is 
covered,  a  photo- 
graphic plate  is 
substituted  for  the 
ground  glass 
screen,  and  then 
the  lens  is  un- 
covered for  a  Cer-  FIG.  192.— The  camera. 
tain  time. 

The  projecting  lantern.  —  The  projecting  lantern  consists 
of  a  light-proof  box,  a  source  of  bright  light,  a  condensing  lens, 
the  lantern  slide,  and  a  projection  lens. 

A  bright  light  is  produced  by  electricity,  or  gas,  or,  as  in  Fig. 
I93>  by  a  lime  light.  The  condenser  consists  of  two  large  con- 
vex lenses.  It  causes  the  strong  light  to  converge  on  the  Ian- 


LENSES  AND  OPTICAL  INSTRUMENTS 


263 


tern  slide.     The  projecting  lens  is  a  system  of  convex  lenses. 
It  throws  an  image  of  the  lantern  slide  on  the  screen. 


FIG.  193.  —  The  projecting  lantern. 

The  eye.  —  A  horizontal  section  of  the  human  eye  is  shown 
in  Fig.  194.  The  covering  of  the  front  of  the  eye  is  a  trans- 
parent membrane  c,  called  the  cornea.  Behind  this  is  a  clear 
liquid  a,  called  the  aqueous  humor.  The  colored  part  of  the 
eye  is  the  iris,  i.  It  is  a  muscular  tissue  with  a  circular  opening, 
the  pupil,  in  the  center.  The 
iris  has  an  involuntary  motion 
which  regulates  the  amount  of 
light  which  enters  the  eye.  It 
enlarges  the  pupil  in  a  dim  light 
and  contracts  it  in  a  bright  light. 
Behind  the  iris  is  a  double  con- 
vex transparent  body  o,  the  crys- 
talline lens.  The  large  cavity  in 
the  eye  is  filled  with  a  jelly-like 
substance  v,  called  the  vitreous 
humor.  The  image  is  formed  on 

the  retina  r,  which  is  an  expansion  of  the  optic  nerve.  The 
optic  nerve  n  carries  the  impression  of  light  from  the  retina  to 
the  brain.  Behind  the  retina  is  a  black  membrane  called 


FIG.  194.  —  The  eye. 


264  PHYSICS  OF  THE  HOUSEHOLD 

the  choroid  coat,  which  incloses  the  interior  cavity  of  the  eye 
except  for  the  opening,  the  pupil.  It  serves  to  keep  all  light 
out  of  the  eye  except  that  which  enters  through  the  pupil ;  it 
also  prevents  interior  reflection  of  the  light  which  enters 
through  the  pupil.  The  outer  coating  of  the  eye  s  is  called  the 
sclerotic;  it  is  the  part  commonly  called  "  the  white  of  the  eye." 

The  eye  resembles  a  camera ;  it  is  a  light-proof  cavity  with  a 
lens  o  in  an  opening  in  one  side,  and  a  screen,  the  retina,  on  the 
other  side.  When  the  eye  is  focused  on  an  object,  the  light 
from  the  object  is  refracted  by  the  crystalline  lens,  the  aqueous 
humor,  and  the  vitreous  humor,  and  an  inverted  image  is  formed 
on  the  retina.  The  image  on  the  retina  is  upside  down  and  it 
would  seem  that  we  should  see  everything  upside  down.  This 
idea  comes  from  the  conception  that  the  brain  is  something 
which  looks  at  the  image  on  the  retina  from  behind,  whereas 
the  light  which  falls  upon  the  retina  simply  stimulates  the  optic 
nerve,  and  this  stimulation  when  carried  to  the  brain  produces 
certain  changes  which  give  us  the  sensation  of  light.  It  is  not 
known  what  changes  are  produced  in  the  brain  by  the  stimulus 
from  the  optic  nerve  nor  how  these  changes  produce  the  sensa- 
tion of  light. 

There  is  one  striking  difference  between  the  eye  and  the 
camera  and  that  is  in  the  method  of  focusing.  The  camera  is 
focused  by  moving  the  lens  back  and  forth,  but  the  eye  is 
focused  by  changing  the  shape  of  the  lens.  When  the  eye  is 
viewing  an  object  near  at  hand,  the  ciliary  muscles  cm  of 
the  eye  contract  the  lens  so  that  it  is  more  convex.  When  it  is 
viewing  a  distant  object,  the  muscles  extend  the  lens  so  that 
it  is  less  convex.  That  is,  when  we  look  at  an  object  near  at 
hand,  the  lens  of  the  eye  is  smaller  and  thicker ;  and  when  we 
look  at  a  distant  object,  it  is  larger  and  thinner. 

Spectacles.  • —  Many  people  can  see  things  at  a  distance  but 
not  those  close  at  hand.  In  such  cases  the  eyes  are  farsighted. 
The  eye  lens  cannot  be  made  thick  enough  to  focus,  on  the 
retina,  light  from  objects  near  at  hand,  and  the  image  would  be 


LENSES   AND   OPTICAL  INSTRUMENTS 


formed  behind  the  retina,  as  shown  in  B,  Fig.  195.  To  overcome 
this  difficulty  spectacles  with  converging  lenses  are  used,  and 
the  proper  lens  is  one  which  will  produce  just  enough  convergence 
to  enable  the  eye  to  focus 
the  image  on  the  retina. 

Some  people  can  see 
things  near  at  hand  but 
not  those  at  a  distance. 
In  such  cases  the  eyes  are 
nearsighted.  The  eye  lens 
cannot  be  made  thin  enough 
to  focus,  on  the  retina,  F 
light  from  a  distant  object, 
and  the  image  is  formed 
in  front  of  the  retina,  see 
A,  Fig.  195.  To  overcome  this  difficulty,  spectacles  with 
diverging  lenses  are  used,  and  the  proper  lens  is  one  which  will 
cause  the  light  to  diverge  to  such  an  extent  that  the  eye  can 
focus  the  image  on  the  retina. 

Magnifying  glass.  —  The  reading  glass,  magnifying  glass,  or 
simple  microscope  is  a  double  convex  lens.     When  it  is  held 


FIG 


Fie.   1 06.  —  The  simple  microscope. 

between  an  object  and  the  eye,  it  enables  the  eye  to  see  an  en- 
larged erect  image  of  the  object.     The  object  must  be  held  at 


266 


PHYSICS  OF  THE  HOUSEHOLD 


a  distance  equal  to  or  less  than  the  focal  length  of  the  lens. 
Such  an  object  is  represented  by  the  arrow  PQ  in  Fig.  196.  In 
this  figure  the  paths  of  three  rays  from  the  top  of  the  arrow  are 
traced.  It  will  be  noticed  that  the  rays  are  still  diverging  after 
they  pass  through  the  lens.  The  rays  from  P  appear  to  the 
eye  to  come  from  p,  those  from  Q  from  g,  and  the  rays  from 
points  between  P  and  Q  appear  to  come  from  points  between 
p  and  q.  Thus  the  eye  sees  an  enlarged  image  right  side  up. 
To  the  eye,  the  light  appears  to  come  from  pq.  The  image  pq 
is  not  a  real  image  because  no  light  comes  from  it.  It  is  a 
virtual  image. 

The   telescope.  —  The   telescope   is   a   combination   of   two 
double  convex  lenses.     The  lens  nearest  the  object  is  called  the 


FIG.  197.  —  The  telescope. 

objective  and  the  lens  nearest  the  eye  is  called  the  eyepiece.  The 
objective  forms  a  real  inverted  image  im  of  the  object  BO  at 
or  near  its  focus.  The  eyepiece  is  so  placed  that  this  image  is 
closer  to  it  than  its  focal  length.  It  magnifies  the  image  and 
the  eye  sees  the  enlarged  virtual  image  IM.  The  telescope 
shown  in  Fig.  197  is  of  the  type  used  in  astronomical  observa- 
tions; in  it  the  image  is  upside  down.  In  the  common  ter- 
restrial telescope  the  image  is  turned  right  side  up  by  means  of 
a  lens  or  combination  of  lenses  placed  between  the  objective 
and  eyepiece. 

The  compound  microscope.  —  The  compound  microscope, 
Fig.  198,  consists  of  two  converging  lenses,  the  objective  and 
the  eyepiece.  The  object  BQ  is  placed  just  outside  the  focal 
length  of  the  objective  and  thus  a  real  enlarged  image  mi  is 


LENSES  AND   OPTICAL  INSTRUMENTS 


267 


formed.     This  image  is  inside  the  focal  length  of  the  eyepiece, 
and  thus  the  eyepiece  forms  an  enlarged  virtual  image  IM. 


FIG.  198. — The  compound  microscope. 

It  will  be  noticed  that  the  object  is  magnified  twice :  first,  by 
the  objective,  and  second,  by  the  eyepiece. 

The  opera  glass.  —  The  opera  glass  is  a  combination  of  two 
lenses ;  the  objective  C  is  a  double  convex  lens  and  the  eye- 
piece c  is  a  double  concave  lens.  The  eyepiece  is  placed 
nearer  to  the  objective  than  the  focal  length  of  the  objective, 


FIG.  199.  —  The  opera  glass. 

see  Fig.  199.  AB  is  the  object  and  if  the  eyepiece  were  absent, 
the  objective  would  form  an  inverted  image  of  AB  at  ab. 
With  the  eyepiece  in  place,  the  rays  are  refracted  and  diverged, 
and  the  rays  which  enter  the  eye  appear  to  come  from  A'B' '. 
A'B'  is  a  virtual  erect  image  of  AB. 

The  stereoscope.  —  The  two  pictures,  placed  side  by  side  on 
the  views  used  with  the  stereoscope,  are  taken  by  two  cameras 
so  placed,  side  by  side,  that  the  pictures  are  not  quite  the  same. 


268 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  200.  —  The 
stereoscope. 


The  glasses  in  the  stereoscope  are  two  pris- 
matic lenses  placed  edge  to  edge  (Fig.  200). 
From  our  study  of  prisms  we  can  understand 
how  the  light  which  enters  the  eye  R  from 
the  picture  A\B\  or  the  eye  L  from  A^B^  is 
so  bent  by  the  prisms  that  it  appears  to  come 
from  A  B  between  the  two.  Each  eye  receives 
a  slightly  different  image,  just  as  it  does  in 
looking  at  objects  without  the  stereoscope, 
and  for  this  reason  the  objects  in  the  picture 
seem  to  stand  out,  just  as  objects  do  when  we 
are  looking  at  them  without  the  stereoscope. 


EXERCISES 

1.  How  do  convex  and  concave  lenses  differ  in  shape? 

2.  Define  real  image,  virtual  image,  principal  axis,  principal  focus, 
and  focal  length. 

3.  Show  by  a  diagram  how  a  convex  lens  produces  a  real  image. 

4.  Show  by  a  diagram  how  a  convex  lens  produces  a  virtual  en- 
larged image. 

5.  Show  by  a  diagram  how  a  concave  lens  produces  a  virtual  reduced 
image. 

6.  Describe  the  camera ;   make  a  diagram. 

7.  Make  a  diagram  of  the  projecting  lantern  and  describe  it. 

8.  Make  a  diagram  of  the  eye  and  describe  it. 

9.  Make  a  diagram  illustrating  longsighted  and  shortsighted  eyes. 
Tell  why  they  are  longsighted  or  shortsighted  and  what  spectacles  are 
required. 

10.  Make  a  diagram  showing  how  the  virtual  image  is  formed  by  a 
magnifying  glass. 

11.  Make  a  diagram  showing  how  the  eye  sees  an  image  in  a  telescope. 

12.  Make  a  diagram  showing  how  the  eye  sees  an  image  in  an  opera 
glass. 

13.  Make  a  diagram  showing  the  path  of  light  in  a  stereoscope.     Ex- 
plain it. 


CHAPTER  XXVII 


COLOR 

Composite  nature  of  white  light.  —  We  can  show  that  white 
light  is  divisible  into  lights  of  different  color  by  means  of  the  ex- 
periment illustrated  in  Fig.  201.    If  we  allow  a  beam  of  white 
light  to  fall  on  a  glass 
prism,   we   find   that 
the    white     light    is 
broken  up  into  lights 
of    the    colors:    red, 
orange,  yellow,  green, 
blue,    indigo,    and 
violet.      This    shows 
that    white    light    is 
composed     of     these 
colors. 

We  can  give  further 
proof  that  white  light 
is  made  up  of  these 
colors  by  the  experi- 
ment illustrated  hi 

Fig.  202.  A  beam  of  white  light  is  separated  into  its  com- 
ponents by  a  glass  prism.  The  resulting  red,  orange,  yellow, 
etc.,  lights  are  allowed  to  fall  on  a  second  prism  placed  in  the 
reverse  position.  The  lights  of  different  color  are  combined 
and  produce  white  light.  This  shows  again  that  white  light 
is  composed  of  lights  of  different  color. 

The  colored  band  produced  by  passing  a  narrow  beam  of 

269 


FIG.  201.  — White  light  divided  into  lights  of 
different  color. 


270 


PHYSICS  OF  THE  HOUSEHOLD 


FIG.  202.  —  White  light  produced  from  lights  of 
different  color. 


white  light  through 
a  prism  is  called 
the  spectrum  of  the 
white  light. 

Lights  of  differ- 
ent colors  have 
different  wave 
lengths.  —  It  was 
stated  above  that 
scientists  believe 
light  to  be  a  wave 

motion  in  the  ether.  The  lengths  of  the  waves  of  light  of  different 
colors  have  been  measured,  and  it  has  been  found  that  they  differ 
in  length.  The  wave  lengths  of  all  colors  are  very  short ;  for 
example,  the  wave  length  of  red  light  is  about  -g^nr  °f  an  mcn> 
and  of  violet  at  the  other  end  of  the  spectrum,  about  half  that 
of  red,  or  -g-OTRT  of  an  inch.  The  wave  lengths  of  other  colors 
fall  between  these.  Waves  a  little  shorter  than  red  are  orange, 
those  a  little  shorter  than  orange  are  yellow,  and  the  wave 
lengths  of  green,  blue,  indigo,  and  violet  are  each  shorter  in 
order.  When  we  look  at  the  spectrum,  we  find  that  one  color 
merges  into  the  next,  and  that  it  is  impossible  to  tell  the  exact 
spot  at  which  one  color  ends  and  the  next  begins.  The  reason 
for  this  is  that  there  are  in  reality  an  infinite  number  of  colors. 
For  example,  what  we  call  red  in  the  spectrum  is  in  reality  made 
up  of  very  many  wave  lengths,  some  longer  than  -s^ruu  of  an 
inch,  and  some  shorter ;  and  each  wave  length  produces  a  shade 
of  red.  The  same  is  true  of  the  other  colors. 

Dispersion.  —  The  bending  of  light  when  it  passes  from  one 
medium  to  another  is  known  as  refraction.  The  spreading  out 
of  white  light,  due  to  the  unequal  bending  of  the  colors  which 
make  up  white  light,  is  known  as  dispersion.  What  is  the  cause 
of  dispersion? 

The  cause  of  refraction,  as  we  learned  above,  is  the  difference 
in  velocity  of  light  in  the  two  media.  The  cause  of  dispersion  is 


COLOR  271 

the  difference  in  velocity  of  different  colors  in  the  same  medium. 
As  far  as  is  known,  all  heat  and  light  waves  travel  with  the  same 
velocity  in  the  ether.  It  has  been  found  by  experiment,  how- 
ever, that  this  is  not  true  in  denser  media,  such  as  glass,  water, 
etc.  In  these  the  long  waves  are  less  retarded  than  the  short 
waves,  and  therefore  the  longer  waves  corresponding  to  red 
color  travel  faster  than  the  shorter  waves  corresponding  to 
violet;  the  other  colors  fall  between  these  two. 

All  the  waves  travel  more  slowly  in  glass  than  in  air,  and 
therefore  all  are  bent  or  refracted  in  passing  from  air  to  glass  or 
the  reverse.  But  in  glass  itself,  the  velocities  are  different, 
and  therefore  the  amount  of  refraction  is  different,  the  violet 
travels  more  slowly  than  the  red  and  therefore  is  more  refracted 
than  the  red.  This  explains  why  the  colors  which  make  up 
white  light  are  spread  out  or  dispersed  by  a  prism. 

Why  objects  are  colored.  —  Now  that  we  know  that  white 
light  is  a  mixture  of  colors,  we  are  in  a  position  to  understand 
why  objects  are  colored.  Let  us  suppose  that  we  are  looking  at 
a  red  dress  in  daylight.  The  dress  is  receiving  light  of  all  colors, 
that  is,  white  light ;  but  it  sends  to  the  eye  only  red  light.  Why 
is  this?  It  has  been  found  that  the  reason  is,  the  dye  in  the 
cloth  absorbs  the  colors  which  are  not  red,  and  therefore  reflects 
only  red.  Similarly  if  we  are  looking  at  a  blue  dress  in  day- 
light, the  cloth  receives  light  of  all  colors  but  reflects  to  the  eye 
only  blue  because  the  dye  in  the  cloth  absorbs  the  other  colors. 
Similarly  with  other  colors.  An  object  seen  in  white  light  is 
colored,  then,  because  it  absorbs  some  of  the  colors  of  white 
light  but  not  others. 

Transparent  substances.  —  We  have  learned  that  in  general 
substances  are  colored  because  certain  of  the  constituents  of 
white  light  are  absorbed.  There  are  many  substances,  however, 
which  allow  light  to  pass  through  without  absorption;  these 
are  said  to  be  transparent;  for  example,  glass,  water,  and 
air. 

A  pane  of  ordinary  window  glass  is  a  very  important  house- 


272  PHYSICS   OF  THE  HOUSEHOLD 

hold  light  appliance.  It  allows  light  to  pass  through,  but  keeps 
out  wind,  rain,  dust,  and  insects.  We  should  soon  realize  the 
importance  of  glass  if  we  had  to  do  without  it.  Before  glass 
was  invented,  windows  were  sometimes  covered  with  cloth,  or 
with  skins  of  animals.  More  often  they  were  simply  holes 
which  were  covered  with  shutters  at  night  or  when  the  weather 
was  stormy. 

Complementary  colors.  —  If  a  beam  of  yellow  light  and  a 
beam  of  blue  light  are  made  to  fall  upon  a  white  screen  at  the 
same  point,  the  resultant  color  is  white.  This  is  also  true  if  the 
beams  are  red  and  bluish  green,  orange  and  greenish  blue,  or 
greenish  yellow  and  violet.  Colors  which  may  be  combined 
in  this  way  to  produce  white  light  are  called  complementary 
colors. 

Mixing  of  pigments.  —  From  the  fact  that  yellow  and  blue 
lights  produce  white,  we  might  expect  that  if  we  mixed  a  yellow 
pigment  with  a  blue  pigment,  we  should  get  white  pigment. 
The  result,  however,  is  a  green  pigment.  The  reason  is,  the 
yellow  pigment  absorbs  red,  blue,  and  violet  from  white  light, 
and  the  blue  pigment  absorbs  red,  orange,  and  yellow  from  white 
light.  Therefore,  the  only  part  of  white  light  which  is  not 
absorbed  by  the  mixture  is  green,  and  the  mixture  sends  green 
light  to  the  eye,  that  is,  it  is  colored  green. 

The  Young-Helmholtz  theory  of  color  vision.  —  The  relation 
of  the  eye  to  color  sensation  has  been  the  subject  of  much  in- 
vestigation, and  the  generally  accepted  theory  of  color  vision 
is  that  proposed  by  Young  and  developed  by  Helmholtz.  It  is 
that  there  are  three  primary  color  sensations  produced  in  the 
eye;  namely,  those  of  red,  green,  and  violet.  The  theory  is 
that  the  retina  is  made  up  of  three  sets  of  nerves,  one  sensitive 
to  red,  one  sensitive  to  green,  and  one  sensitive  to  violet.  When 
all  are  stimulated  at  the  same  time,  the  result  is  the  sensation 
of  white  light.  When  only  one  or  two  are  stimulated,  the  result 
is  colored  light. 


COLOR  273 

EXERCISES 

1.  Making  a  drawing  showing  how  white  light  is  spread  out  into  a 
spectrum  by  a  glass  prism. 

2.  Tell  how  lights  of  different  colors  differ  in  wave  lengths. 

3.  Explain  why  a  prism  spreads  out  the  colors  of  white  light. 

4.  Explain  why  a  dress  is  red,  blue,  or  yellow. 

5.  Explain  why  colored  pigments  mixed  do  not  give  the  same  color 
as  lights  of  the  same  colors  mixed. 

6.  State  the  Young-Helmholtz  theory  of  color  vision. 


CHAPTER  XXVIII 
SOUND 

How  sound  is  produced.  —  In  this  chapter  and  the  one  fol- 
lowing we  shall  study  sound,  particularly  the  sound  obtained 
from  musical  instruments.  Let  us  consider,  first,  how  sound  is 
produced. 

In  the  stringed  instruments,  such  as  the  banjo,  guitar,  violin, 
and  piano,  the  vibrations  of  the  strings  set  the  body  of  the 
instrument  or  the  sounding  board  in  vibration,  and  this  produces 
the  sound. 

In  the  wind  instruments,  such  as  the  trumpet,  cornet,  and 
trombone,  the  vibration  of  the  player's  lips  sets  the  air  in  the 
instrument  in  vibration,  and  this  produces  the  sound. 

In  the  case  of  the  voice,  the  vibration  of  the  vocal  chords 
sets  the  air  in  the  throat  and  mouth  in  vibration,  and  this 
produces  the  sound. 

In  fact,  in  every  case,  sound  is  produced  by  the  movement  of 
some  material  body. 

Sound  travels  through  gases,  liquids,  and  solids.  —  A  few 
simple  experiments  will  demonstrate  that  sound  travels  through 
gases,  liquids,  and  solids,  as  follows : 

It  is  hardly  necessary  to  make  a  special  experiment  to  show 
that  sound  travels  through  air,  since  this  is  demonstrated  every 
time  we  hear  a  person  speak.  Sound  travels  through  all  gases. 

If  you  put  your  head  under  water  and  make  a  sound  of  some 
sort  in  the  water,  for  example,  by  striking  two  stones  together, 
you  hear  the  sound  very  distinctly.  This  illustrates  the  fact 
that  sound  travels  through  liquids. 

274 


SOUND  275 

If  a  long  piece  of  wood  is  held  against  the  ear  and  a  person 
scratches  the  other  end  lightly  with  a  pin,  the  sound  is  heard 
very  distinctly.  Also,  if  a  spoon  is  suspended  from  a  string  and 
the  end  of  the  string  is  wound  around  the  finger  placed  against 
the  ear,  the  sound  heard  when  the  spoon  is  struck  is  very  dis- 
tinct. These  illustrate  the  fact  that  sound  travels  through 
solids. 

Sound  does  not  travel  through  a  vacuum.  —  Suspend  an 
electric  bell  in  the  bell  jar  of  an  air  pump,  Fig.  203,  and  pass  an 
electric  current  through  the  bell  while  the 
jar  is  full  of  air;  a  loud  sound  is  heard. 
Produce  a  vacuum  in  the  jar  and  again  pass 
the  current  through  the  bell ;  a  very  slight 
sound  is  heard,  and  the  better  the  vacuum 
the  less  the  sound.  This  illustrates  the  fact 
that  sound  does  not  travel  through  a 
vacuum.  In  this  experiment  a  certain 
amount  of  sound  is  heard  because  the 
vibration  of  the  bell  is  carried  along  the 
wires  to  the  sides  of  the  bell  jar  and  thus  to 
the  outer  air.  FIG.  203.  —  Sound  does 

The  velocity  of  sound.  —  The  velocity  of      not  travel  throu«h  a 

J  vacuum. 

sound  in  air  is  found  to  be  1087  ft.  per 
second  at  o°  C.,  and  1126  ft.  per  second  at  20°  C.  (68°  F.). 
The  velocity  increases  not  quite  2  ft.  per  second  for  each 
increase  in  temperature  of  i°  C.  (In  our  calculations  we  may 
use  i  loo  ft.  per  second  for  the  velocity  of  sound  in  air.)  In 
water  the  velocity  of  sound  at  o°  C.  is  4590  ft.  per  second,  and 
in  iron  16,728  ft.  per  second. 

How  sound  travels.  —  When  a  gun  is  discharged,  the  air 
nearest  the  muzzle  is  shoved  out  in  all  directions.  This  air 
pushes  back  the  air  farther  away,  and  this  air  in  turn  moves  air 
farther  away,  and  so  on.  In  this  way  a  spherical  compression 
wave  is  sent  out  in  all  directions. 

Each  particle  of  air  moves  only  a  short  distance  out  and  back 


276  PHYSICS  OF  THE  HOUSEHOLD 

and  then  comes  to  rest,  but  the  compression  moves  out  indef- 
initely. This  is  true  also  when  sound  travels  through  solids 
or  liquids. 

When  a  gun  is  discharged,  only  one  wave  is  produced,  and 
therefore  only  one  sound  is  heard.  When,  on  the  contrary, 
a  bell  is  struck  once  by  the  clapper,  it  continues  to  vibrate 
and  sends  out  a  series  of  spherical  waves  (Fig.  204).  These 


FIG.  204.  —  Illustrating  sound  waves. 

waves  travel  out  in  all  directions,  and  when  they  fall  upon  the 
ear  produce  a  series  of  sounds. 

What  is  sound  ?  —  A  sound,  then,  may  be  denned  as  any 
vibration,  in  a  solid,  liquid,  or  gas,  which  has  sufficient  energy 
to  affect  the  ear. 

Nature  of  sound  waves.  —  Each  sound  wave  consists  of  a 
compression  and  a  rarefaction.  This  is  illustrated  in  Fig.  204; 
the  dark  circles  represent  the  compressions  and  the  lighter 
circles  the  rarefactions.  The  compressions  are  produced  by 
the  outward  movement  of  the  metal  of  the  bell,  the  rarefactions 
by  the  inward  movement  of  the  metal. 

If  a  line  were  drawn  from  the  bell  to  the  ear  in  this  figure,  it 
would  pass  through  a  rarefaction,  a  compression,  a  rarefaction, 
a  compression,  etc.  The  length  of  one  rarefaction  and  one 
compression  on  such  a  line  is  one  wave  length.  In  Fig.  211 
the  distance  be  is  one  wave  length. 


SOUND 


277 


Wave  length.  —  Sound  travels  at  the  rate  of  about  1 100  ft. 
per  second.  If  a  bell  makes  100  vibrations  per  second,  it  pro- 
duces 100  waves  per  second  and  each  wave  is  -yVrf  =  II  ft. 
long.  If  it  makes  1000  vibrations  per  second,  it  produces  1000 
waves  and  each  wave  is  i.i  ft.  long. 

If  we  let  v  =  velocity  of  sound,  n  =  the  number  of  vibra- 
tions per  second  of  the  source  of  the  sound,  and  /  =  the  length 
of  the  sound  waves  produced  by  this  source,  then : 


Noises  and  musical  notes.  —  The  difference  between  noises 
and  musical  notes  can  be  illustrated  as  follows:  If  a  card  is 
held  against  the  teeth  of  a  revolving  toothed  wheel,  on  which 
the  teeth  are  placed  at  irregular  intervals,  the  sound  heard  is 
disagreeable,  that  is,  it  is  a  noise.  If  the  card  is  held  against 
the  teeth  of  a  revolving  toothed  wheel  on  which  the  teeth  are 
placed  at  regular  intervals,  the  sound  heard 
is  a  musical  note. 

This  illustrates  the  difference  between 
noises  and  musical  notes.  A  noise  is  a 
single  sound  or  succession  of  sounds  at  ir- 
regular intervals.  A  musical  note  is  a  series 
of  sounds  in  rapid  succession  at  regular  in- 
tervals. If  the  sounds  at  regular  intervals 
are  less  frequent  than  40  per  second,  they 
are  not  musical  and  are  classed  as  noise. 
The  frequencies  used  in  music  are  from 
about  40  to  4000  vibrations  per  second. 

The  pitch  of  a  note  depends  on  the 
number  of  waves  per  second.  —  This  can 
be  shown  by  means  of  the  disk,  Fig.  205. 
The  disk  is  perforated  with  holes  at  regular  FIG.  205.— The  pitch  of 
intervals.  A  blast  of  air  is  directed  against 
the  disk  in  such  a  way  that  a  puff  goes  per  second. 


278  PHYSICS  OF  THE  HOUSEHOLD 

through  each  hole  as  it  passes  the  tube.  It  is  found  that  as  the 
disk  is  revolved  more  and  more  rapidly  the  pitch  of  the  note  pro- 
duced becomes  higher.  Since  a  wave  is  started  by  each  puff 
and  the  greater  the  number  of  revolutions  per  second  the  greater 
the  number  of  puffs  per  second,  we  conclude  that  the  greater  the 
number  of  waves  per  second  the  higher  is  the  pitch  of  the  note 
produced. 

Loudness.  —  The  loudness  of  a  sound  depends  on  the  energy 
given  to  the  air  by  the  source  of  the  sound,  and  upon  the  dis- 
tance from  the  source.  The  sound  from  any  given  source 
decreases  in  loudness  as  we  move  away  from  the  source.  Sound 
waves  are  spherical  and  therefore  at  a  distance  of  10  ft.  from  the 
source,  they  cover  four  times  the  surface  they  do  at  a  distance 
of  5  ft.  from  the  source.  Thus  the  sound  at  10  ft.  is  only  one 
fourth  as  loud  as  it  is  at  5  ft.,  that  is,  the  loudness  varies  inversely 
as  the  square  of  the  distance  from  the  source. 

Echo.  —  An  echo  is  caused  by  sound  waves  reflected  from  some 
large  object  at  a  distance. 

EXERCISES 

1.  How  is  sound  produced? 

2.  Through  what  does  sound  travel? 

3.  Describe  sound  waves. 

4.  What  is  the  length  of  the  sound  waves  produced  by  a  person  whose 
vocal  chords  vibrate  200  times  a  second? 

5.  What  is  the  difference  between  noise  and  music  ? 

6.  On  what  does  the  pitch  of  a  note  depend? 

7.  On  what  does  the  loudness  of  a  sound  depend? 


CHAPTER  XXIX 


MUSIC   AND   MUSICAL  INSTRUMENTS 

Musical  scale.  —  The  disk  shown  in  Fig.  206  has  four  rows 
of  holes.  The  inside  row  has  24  holes,  the  next  30,  the  next 
36,  and  the  outside  row  48.  If  this  disk  is  revolved  and  a  jet 
of  air  is  directed  against  each 
row  of  holes  in  turn,  beginning 
with  the  inside  row,  the  notes 
do,  mi,  sol,  do'  are  obtained. 

When  the  air  jet  passes 
through  a  hole,  it  produces  a 
wave  in  the  air  on  the  opposite 
side  of  the  disk.  We  learn  from 
this  experiment  that  the  num- 
ber of  waves  per  second  which 
produce  the  notes  do,  mi,  sol,  do' 
are  in  the  ratio  24,  30,  36,  48. 

We  can    produce    the    eight 

notes  of  the  octave  by  using  a  disk  with  eight  rows  of  holes, 
the  numbers  of  holes  being  24,  27,  30,  32,  36,  40,  45,  and  48. 

The   notes,  the  corresponding  number  of  holes,  and  their 
ratios  are : 
Notes 

ratio 
ratio 

This  means,  not  that  the  notes  of  a  musical  scale  are  pro- 
duced by  24,  27,  30,  etc.,  vibrations  per  second,  but  that  the 
vibrations  per  second  are  in  this  ratio. 

279 


FIG.  206.  —  The  notes  do,  mi,  sol,  do'. 


c 

D 

E 

F 

G 

A 

B       C 

do 

re 

mi 

fa 

sol 

la 

si     do' 

24 

27 

30 

32 

36 

40 

45      48 

i 

1 

f 

I 

f 

1 

V        * 

280  PHYSICS  OF  THE  HOUSEHOLD 

If  we  wish  to  determine  the  musical  scale  of  a  note  of  any 
frequency,  we  proceed  as  follows :  call  the  note  do,  then  re  is 
produced  by  f  the  frequency  of  do ;  mi  by  J ;  fa  by  J  the  fre- 
quency of  do,  etc. 

It  will  be  noticed  that  the  note  one  octave  higher  than  any 
note  do  is  produced  by  just  twice  as  many  waves  per  second 
as  do. 

Stringed  instruments.  —  Let  us  now  use  the  knowledge  we 
have  gained  of  pitch  and  the  musical  scale  to  help  us  under- 
stand stringed  instruments.  Many  of  us  have  had  experience 
with  one  or  more  of  these  instruments,  for  example,  with  the 
guitar,  violin,  banjo,  or  piano.  We  know  that,  in  general,  the 
short,  light  strings  give  notes  of  high  pitch  and  the  long,  heavy 


FIG.  207.  —  The  sonometer. 

strings  notes  of  low  pitch.  We  know  also  that  tightening  a 
string  raises  its  pitch  and  loosening  it  lowers  its  pitch. 

We  can  make  an  instructive  experiment  with  any  string  of  a 
banjo,  guitar,  or  violin  as  follows :  Measure  the  length  of  the 
string  and  sound  it,  call  the  note  do.  Then  make  the  string  in 
succession  f ,  f  and  \  as  long  and  sound  it.  The  notes  produced 
are  fa,  sol,  do',  which  we  know  are  produced  by  f,  f ,  and  2  times 
as  many  vibrations  per  second  as  do.  That  is,  the  rate  of 
vibration  of  a  string  varies  inversely  as  the  length  of  the 
string. 

Vibrating  strings  are  studied  by  means  of  the  sonometer 
shown  in  Fig.  207.  With  this  instrument  the  length  and  ten- 
sion of  the  strings  can  be  varied  easily,  and  the  following  laws 
can  be  illustrated. 


MUSIC  AND   MUSICAL  INSTRUMENTS  281 

Laws  of  vibrating  strings.  —  The  rate  of  vibration  of  a  string 
varies : 

(1)  inversely  as  the  length, 

(2)  directly  as  the  square  root  of  the  stretching  force, 

(3)  inversely  as  the  thickness, 

(4)  inversely  as  the  square  root  of  the  density  of  the  material. 
Sounding  board.  —  The  sound  produced  by  a  piano  comes 

from  the  sounding  board.  The  strings  set  the  sounding  board 
in  vibration  and  these  vibrations  give  the  sound.  In  the  case 
of  the  guitar,  violin,  harp,  the  body  of  the  instrument  acts  as 
the  sounding  board  and  produces  the  sound. 

The  action  of  a  sounding  board  can  be  illustrated  by  means 
of  a  tuning  fork.  If  a  tuning  fork  is  set  in  vibration  and  held 
in  the  hand,  the  sound  produced  is  not  great.  If,  however,  the 
handle  of  the  fork  is  placed  against  a  large  elastic  surface,  such 
as  the  panel  of  a  door  or  the  top  of  a  table,  the  sound  is  much 
louder.  The  panel  or  table  is  set  in  vibration  by  the  fork  and, 
being  much  larger  in  area  than  the  prongs  of  the  fork,  sets  a 
much  larger  mass  of  air  in  vibration ;  thus  it  gives  a  louder 
sound.  The  sounding  board  is  the  important  part  of  any 
stringed  instrument ;  for  example,  one  violin  is  more  valuable 
than  another,  not  because  of  any  difference  in  the  strings, 
but  because  the  body  of  one  gives  out  fuller,  richer  notes 
than  that  of  the  other. 

Fundamental,  octaves,  and  overtones.  —  The  lowest  note 
that  can  be  produced  by  a  vibrating  body,  such  as  a  stretched 
string  or  an  air  column,  is  called  the  fundamental  of  the  vibrat- 
ing body. 

A  note  one  octave  higher  than  the  fundamental  is  made  up  of 
twice  as  many  waves  per  second  (see  page  279).  A  note  one 
octave  higher  still  is  made  up  of  twice  as  many  waves  per  sec- 
ond as  the  first  octave,  or  four  times  as  many  waves  as  the 
fundamental.  The  note  the  third  octave  higher  is  made  up  of 
twice  as  many  waves  per  second  as  the  second  octave,  or  eight 
times  as  many  as  the  fundamental,  and  so  on. 


282  PHYSICS  OF  THE  HOUSEHOLD 

If  we  consider  the  fundamental  note  to  be  made  up  of  100 
waves  per  second,  then  the  number  of  waves  in  the  octaves  are 
in  order  as  follows : 

fundamental          ist  octave          2d  octave  3d  octave          4th  octave 

100  200  400  800  1600 

The  overtones  or  harmonics  of  a  fundamental  are  the  notes 
produced  by  2,  3,  4,  5,  6,  etc.,  times  as  many  waves  per  second 
as  the  fundamental.  For  example,  if  we  consider  the  funda- 
mental to  be  produced  by  100  vibrations  per  second,  the  number 
of  waves  per  second  in  the  overtones  are  in  order  as  follows : 

fundamental      ist  overtone        2d  overtone        3d 'overtone      4th  overtone 
loo  200  300  400  500 

It  will  be  noticed  that  all  octaves  are  overtones,  but  that  all 
overtones  are  not  octaves.  This  will  help  us  to  distinguish 
between  octaves  and  overtones,  which  are  sometimes  confused. 

Quality.  —  We  can  readily  distinguish  a  note  played  on  one 
instrument  from  the  same  note  played  on  any  other  instrument. 
For  example,  middle  C  on  the  piano  is  easily  distinguished  from 
middle  C  on  the  violin,  organ,  or  flute,  and  each  of  these  is 
easily  distinguished  from  the  others.  Why  is  this?  If  they 
are  the  same  note,  why  is  there  a  difference  ?  These  notes  are 
said  to  differ  as  to  the  quality  of  tone.  In  order  to  understand 
differences  in  quality  it  will  be  necessary  to  study  fundamentals 
and  overtones  a  little  further. 

When  a  wire  vibrates  as  a  whole  as  shown  in  A,  Fig.  208,  it 
produces  its  fundamental  note.  If  each  half  of  the  wire  vi- 
brates as  shown  in  B,  it  produces  its  first  overtone,  which  has 
twice  as  many  vibrations  per  second  as  the  fundamental.  If 
each  third  of  the  wire  vibrates  as  shown  in  C,  the  note  produced 
is  the  second  overtone. 

The  actual  vibrations  of  a  wire  are  always  complex.  They 
are  made  up  of  those  which  produce  the  fundamental,  and  those 
which  produce  various  overtones.  Thus  a  fundamental  tone 
is  never  pure,  no  matter  what  instrument  produces  it ;  it  is 


MUSIC  AND   MUSICAL   INSTRUMENTS 


283 


always  accompanied  by  overtones.  The  overtones,  however, 
differ  in  loudness  according  to  the  instrument.  Every  tone, 
then,  is  made  up  of  a  fundamental  and  many  overtones,  and 
the  same  notes  in  different  instruments  differ  in  quality  because 
the  fundamental  is  accompanied  by  overtones  which  vary  in  number 
and  loudness. 

Let  us  explain  this  a  little  further.  The  sound  we  hear 
from  a  violin,  for  example,  really  comes  from  the  wood;  the 
wood  is  forced  to  vibrate  at  the  same  rate  as  the  string :  these 
are  called  forced  vibrations.  However,  each  piece  of  wood 
vibrates  more  readily  with  cer- 
tain tones  than  with  others,  so 

that  when  it  is  set  vibrating  by  A.  FUNDAMENTAL  1:1 

a  string,  those  overtones  which 
it  produces  most  readily  are 
louder  than  the  others.  The 
quality  of  the  resulting  tone  de- 
pends upon  which  of  the  over- 
tones happen  to  be  loud.  In  a 
piano  the  sound  we  hear  comes 
from  the  sounding  board;  in 

the  guitar,  mandolin,  etc.,  from  the  wooden  body ;  and  in  wind 
instruments  from  the  vibrating  air  column.  In  each  case  cer- 
tain overtones  are  produced  more  readily  than  others,  and  the 
quality  of  the  resulting  tones  varies  for  this  reason. 

Wind  instruments.  —  In  wind  instruments  the  air  column  in 
the  instrument  is  set  in  vibration  by  some  means,  and  these 
vibrations  produce  the  musical  note. 

The  pitch  of  the  note  produced  depends  on  the  strength  of 
the  air  blast  and  on  the  length  of  the  air  column. 

For  a  given  air  column,  gentle  blowing  produces  either  the 
fundamental  note  or  one  of  the  low  overtones ;  harder  blowing 
produces  the  higher  overtones. 

For  a  given  strength  of  air  blast,  the  pitch  varies  inversely 
as  the  length  of  the  air  column. 


B.     FIRST  OVERTONE    2:1 


C.     SECOND  OVERTONE    3:1 

FIG.  208.  —  How  overtones  are 
produced. 


284 


PHYSICS  OF  THE  HOUSEHOLD 


We  can  illustrate  this  nicely  with  the  common  tin  whistle 
or  flageolet  or  with  a  fife.  With  all  the  holes  closed,  gentle 
blowing  produces  the  fundamental,  harder  blowing  the  over- 
tones. We  can  obtain  the  eight  notes  of  the  octave  as  follows. 
Start  with  the  note  given,  when  all  the  holes  are  closed  and 
the  air  blast  has  a  certain  strength;  call  this  note  do.  We 
obtain  the  next  six  notes  by  opening  the  holes  in  order,  beginning 
at  the  lower  end.  Opening  the  holes  shortens  the  length  of  the 
air  column.  The  eighth  note  is  obtained  by  closing  all  the 
holes  and  blowing  a  little  harder. 

How  the  sound  is  started.  —  The  air  column  in  a  wind  instru- 
ment is  set  in  vibration  in  various  ways :  by  the  vibrations  of 
the  player's  lips,  by  the  vibrations  of  a  reed,  or  by  the  vibra- 
tions of  an  air  blast  directed  across  a  thin  edge.  The  vibrations 
produced  in  these  different  ways  are  complex,  that  is,  they  are 
made  up  of  vibrations  of  different  frequencies.  The  air  column, 
however,  vibrates  in  unison  with  only  one  frequency,  depending 
on  its  length  and  on  the  strength  of  the  air  blast. 

In  the  case  of  the  fife,  for 
example,  the  air  blast  is  directed 
across  the  edge  of  a  hole.  This 
produces  a  fluttering  which  is 
made  up  of  vibrations  of  dif- 
ferent frequencies.  The  air 
column  in  the  fife  vibrates  in 
unison  with  one  of  these  fre- 
quencies, which  frequency  de- 
pends on  the  length  of  the  air 
column  and  on  the  strength  of 
the  air  blast. 

Resonance.  —  If    a    tuning 
fork  is  sounded  and  held  over 
a  closed  air  column  (AB,  Fig. 
209)  which  is  just  J  as  long  as 
FIG.  209.  — Resonance.  the  wave  length  produced  by 


MUSIC  AND  MUSICAL  INSTRUMENTS  285 

the  fork,  a  loud  sound  is  heard.  The  fork  sets  the  air-column 
in  vibration  and  these  vibrations  produce  the  loud  sound. 
This  is  an  example  of  resonance. 

We  can  explain  resonance  as  follows :  If  we  are  swinging  a 
person,  we  can  keep  the  swing  in  motion  with  little  effort  if  we 
give  it  gentle  pushes  at  just  the  right  time ;  that  is,  if  the  rate 
of  vibration  of  the  swing  and  the  rate  of  applying  the  pushes  are 
exactly  the  same. 

Now  the  rate  of  vibration  of  the  fork  used  above  is  exactly 
the  same  as  the  rate  of  vibration  of  the  air  column.  The  fork, 
therefore,  gives  impulses  to  the  air  column  at  just  the  right 
times  and  thus  sets  it  in  vibration. 

Interference.  —  If  we  produce  resonance  as  above,  and  then 
revolve  the  prongs  of  the  fork  slowly,  we  find  that  there  is 
silence  when  the  prongs  are  at  an 

angle  of  45°  to  the  top  of  the  air      N  ,' 

column.    In  this  case  the  compres-  \  /' 

sion  produced  by  one  side  of  the 
prong  is  destroyed  by  the  rarefac- 
tion produced  by  the  other  side, 
that  is,  the  two  sound  waves  pro-  / 

duce  silence.     This  is  known  as  / 

interference.  ' 

The  explanation  of  this  silence       /' 
is  illustrated  in  Fig.  210.     A  and          FlG.  2I0.- interference. 
B  are  the  ends  of  the  prongs  of  a 

tuning  fork.  These  ends  are  moving  in  the  direction  shown 
by  the  arrows ;  they  are  producing  compressions  in  the  direc- 
tions cd  and  ce,  and  rarefactions  in  the  directions  /  and  g. 

Along  the  dotted  lines  the  compressions  and  rarefactions 
touch.  At  any  point  on  these  lines,  then,  one  wave  is  trying 
to  compress  the  air,  and  the  other  is  trying  to  rarefy  it.  The 
result  is  that  there  is  neither  compression  nor  rarefaction ;  that 
is,  the  air  is  still  and  there  is  no  sound.  This  is  an  example  of 
two  series  of  sound  waves  which  interfere  and  produce  silence. 


\  <  / 

^  A     B/ 

M  c  Q 
* 


^ 


286  PHYSICS  OF  THE  HOUSEHOLD 

Harmony  and  discord.  —  Whether  two  notes  are  harmonious 
or  discordant  depends  on  how  many  beats  they  produce  per 
second.  To  understand  this  it  is  necessary  to  understand  beats. 
We  can  illustrate  beats  as  follows. 

Tune  two  strings  of  a  guitar  or  other  stringed  instrument,  to 
the  same  pitch,  then  change  the  pitch  of  one  slightly  and  sound 
them  together.  It  is  found  that  the  sound  varies  in  loudness 
at  short  intervals  (it  sounds  a  little  as  though  a  person  were 
saying  wa,  wa,  wa,  wa).  These  variations  in  sound  are  known 
as  beats.  These  beats  may  be  shown  also  as  follows :  Take  two 
tuning  forks  of  the  same  pitch ;  stick  some  wax  on  the  prongs  of 
one  so  that  they  are  heavier  and  vibrate  a  little  more  slowly, 
then  sound  the  forks  together.  It  is  found  again  that  beats 
are  produced. 

Explanation  of  beats.  —  We  can  represent  a  sound  wave  by  a 
curved  line  as  shown  in  Fig.  211.  Let  the  hills  a  represent  the 
a  o  compressions,  and  the  hollows 

b,  c,  etc.,  the  rarefactions. 

FIG.  ^.-Representing  soundwaves.          Let   US   SUPPOSC   that  WC  are 

listening  to  two  notes  of  nearly 

the  same  number  of  vibrations  per  second.  Let  us  suppose,  for 
simplicity,  that  one  note  makes  just  one  vibration  per  second 
more  than  the  other.  The  two  series  of  waves  can  be  represented 
as  shown  in  Fig.  212.  If  at  the  beginning  of  a  second  the  waves 
are  both  compressing  or  rarefy  ng  the  air,  we  hear  a  sound  twice 
as  loud  as  that  produced  by  one  note.  At  the  end  of  J  second, 
one  note  is  compressing  the  air,  while  the  other  is  rarefying 
it.  The  result  is  the  air 
is  neither  compressed  nor 
rarefied  and  there  is  si- 

FIG.  212.  —  How  beats  are  produced  by 
lence.      At  the  end  of  the  interference. 

second    both    notes    are 

again  compressing  or  rarefying  the  air,  and  the  sound  is  twice 
as  loud  as  that  produced  by  one  note.  Thus  there  is  one 
silence  and  one  loud  sound  each  second,  when  one  note  makes 


MUSIC  AND   MUSICAL  INSTRUMENTS  287 

one  vibration  per  second  less  than  the  other ;  that  is,  there  is 
one  beat  per  second. 

If  one  note  makes  two  vibrations  less  per  second  than  the 
other,  there  are  two  silences  and  two  loud  sounds  per  second; 
that  is,  there  are  two  beats  per  second,  etc.  This  shows  how 
beats  are  produced  by  interference. 

Two  notes  are  discordant  when  they  produce  a  certain  num- 
ber of  beats  per  second.  The  discord  is  greatest  when  there 
are  about  33  beats  per  second.  When  there  are  fewer  than  6 
or  8  or  more  than  70  or  80  the  roughness  disappears  and  the 
notes  are  harmonious.  The  beats  may  be  produced  by  the 
fundamentals,  or  by  the  fundamental  of  one  and  an  overtone 
of  the  other,  or  by  an  overtone  of  one  and  an  overtone  of  the 
other.  Whether  notes  are  harmonious  or  discordant,  then, 
depends  upon  the  number  of  beats  produced  per  second. 

The  phonograph.  —  One  of  the  most  wonderful  of  sound 
instruments  is  the  phonograph,  invented  by  Thomas  A.  Edison. 
The  principle  of  the  machine  is  simple. 
A  small  stylus  G  is  attached  to  the  center 
of  a  flexible  disk  B1  Fig.  213,  which  is 
supported  at  the  edge.  The  point  of  the 
stylus  bears  on  the  wax  cylinder  C. 

In  making  a  record  a  person  speaks  or 
plays   an   instrument,   in    front   of    the 
mouthpiece   A.    The  sound   waves   set 
the   disk   in   vibration,  and   the   stylus      FlG-  213.  — Thephono- 
under  the  disk  digs  into  the  wax  cylinder. 

The  cylinder  is  kept  revolving  at  a  certain  rate,  and  the  disk 
and  stylus  move  sidewise  slowly;  thus  the  stylus  traces  a 
spiral  groove  on  the  cylinder.  Each  wave  of  sound  produces 
one  vibration  of  the  disk  and  stylus,  and  for  each  vibration  the 
stylus  makes  a  small  hollow  and  hill  in  the  groove.  If  the 
groove  is  examined  with  a  magnifying  glass  it  is  found  to  con- 
sist of  a  series  of  irregular  hills  and  hollows. 

In  reproducing  the  sound,  the  stylus  is  placed  at  the  starting 


288  PHYSICS   OF  THE  HOUSEHOLD 

point  and  the  cylinder  is  revolved  at  the  same  rate  as  when  the 
record  was  being  made.  The  stylus  traveling  along  the  groove 
moves  down  into  the  hollows  and  up  over  the  hills,  and  gives 
the  disk  this  up  and  down  motion,  with  the  result  that  the  disk 
makes  the  same  vibrations  that  it  did  when  the  record  was 
being  made.  These  vibrations  give  the  air  in  the  mouthpiece 
the  same  vibration,  and  therefore  the  sounds  heard  are  those 
which  were  used  in  making  the  record. 

EXERCISES 

1.  State  the  ratios  of  the  number  of  vibrations  per  second  of  the  notes 
of  the  musical  scale. 

2.  State  the  laws  of  vibrating  strings. 

3.  What  is  the  function  of  a  sounding  board? 

4.  Define  fundamental,  octave,  and  overtone. 

5.  Why  do  notes  differ  in  quality? 

6.  Explain  how  beats  are  produced. 

7.  Why  are  notes  discordant? 

8.  Describe  the  phonograph. 


CHAPTER  XXX 
A  FURTHER  STUDY  OF  MECHANICS 

GRAVITATION,  COMPOSITION  or  FORCES 

Gravitation.  —  It  was  stated  on  page  28  that  the  weight  of 
a  body  is  the  measure  of  the  earth's  attraction  for  it.  Let  us 
consider  this  a  little  further.  If  we  hold  in  the  hand  an  object 
such  as  a  book,  a  stone,  or  a  flatiron,  we  feel  a  certain  pull 
towards  the  earth.  If  we  release  the  object,  it  falls  to  the  ground. 
We  account  for  these  facts  by  saying  that  the  earth  exerts  an 
attractive  force  upon  the  object.  We  call  this  force  gravita- 
tion or  gravity. 

Sir  Isaac  Newton  studied  the  action  of  gravitation  as  shown 
by  falling  bodies  and  as  shown  by  the  motion  of  the  planets. 
He  was  led  to  certain  conclusions,  which  he  stated  as  follows : 
"  Every  particle  of  matter  in  the  universe  exerts  an  attraction 
on  every  other  particle.  This  attraction  varies  directly  as  the 
product  of  the  masses  of  the  particles  and  inversely  as  the  square 
of  the  distance  between  them."  This  is  known  as  the  Law  of 
Universal  Gravitation. 

Center  of  gravity.  —  The  earth  exerts  an  attractive  force 
upon  every  particle  of  a  body  and  the  sum  of  all  these  small 
attractive  forces  is  the  weight  of  the  body.  //  is  possible  to 
find  one  point  such  that  if  a  force  equal  to  the  weight  of  the  body 
is  exerted  upwards  at  that  point,  the  body  is  in  equilibrium. 
This  point  is  called  the  center  of  gravity. 

Equilibrium.  —  A  body  may  be  in  stable,  unstable,  or  neutral 
equilibrium.    A  body  is  said  to  be  in  stable  equilibrium  if 
u  289 


290  PHYSICS  OF  THE  HOUSEHOLD 

it  returns  to  its  original  position  when  slightly  displaced  (Aj 
Fig.  214). 

A  body  is  said  to  be  in  unstable  equilibrium  if,  after  being 
slightly  displaced,  it  tends  to  move  farther  from  its  original 
position  (B,  Fig.  214). 

A  body  is  said  to  be  in  neutral  equilibrium  if,  when  slightly 
displaced,  it  moves  neither  back  to  its  original  position  nor  farther 

away  from  it  (C,  Fig.  214). 

A  body  will  stand  up  as 
long  as  its  center  of  gravity 
is  supported ;  that  is,  as  long 
as  the  line  drawn  vertically 
downwards  through  its  center 

FIG.  214.  —  Examples  of  stable,  unstable.        r  v        f  n       •      *j       *u 

and  neutral  equilibrium.  °f     Sravlty    falls     mside     the 

base  of  the  body. 

Composition  of  forces.  —  The  single  force  which  has  the  same 
effect  as  two  or  more  forces  is  called  the  resultant  of  these  forces. 
The  single  force  which  keeps  one  or  more  forces  in  equilibrium 
is  called  the  equilibrant  of  these  forces. 

If  two  or  more  forces  are  acting  at  the  same  point  and  in  the 
same  direction,  the  resultant  is  equal  to  the  sum  of  the  forces 
and  it  acts  in  the  same  direction ;  the  equilibrant  is  equal  to 
the  sum  of  the  forces  and  acts  in  the  opposite  direction. 

Forces  acting  at  an  angle  to  each  other.  —  If  two  forces 
are  acting  at  the  same  point  but  at  an  angle  to  each  other,  their 
resultant  and  equilibrant  are  not  equal  to  the  sum  of  the  two 
forces.  There  is,  however,  a  simple  rule  by  which  these  may 
be  found. 

The  rule  is  as  follows :  "  //  two  forces  acting  at  an  angle 
upon  a  point  are  represented  in  direction  and  amount  by 
straight  lines,  the  resultant  of  the  two  forces  is  exactly  repre- 
sented in  direction  and  amount  by  the  diagonal  of  the  parallel- 
ogram of  which  the  lines  are  the  sides.  The  equilibrant  is 
equal  to  the  resultant  but  is  in  the  opposite  direction"  This 
is  known  as  the  Parallelogram  Law  of  Forces. 


A  FURTHER  STUDY  OF  MECHANICS 


2QI 


D' 

FIG.  215.  —  Illustrating 
the  parallelogram  law 
of  forces. 


In  Fig.  215,  let  the  length  of  AC  represent 
the  amount  of  one  force  and  the  direction 
of  AC  the  direction  of  this  force,  and  let 
the  length  of  A  B  and  its  direction  represent 
the  amount  and  direction  of  another  force  ; 
then  A  D  represents  the  resultant  in  amount 
and  direction  and  AD'  represents  the  equili- 
brant in  amount  and  direction. 

Parallel  forces.  —  //  two  forces  are  acting 
on  a  body  at  different  points  but  in  the  same 
direction,  the  forces  are  parallel.  The  re- 
sultant is  equal  to  the  sum  of  the  forces  and 
acts  in  the  same  direction.  The  equilibrant 
is  equal  to  the  sum  of  the  forces  and  acts 
in  the  opposite  direction.  The  point  of  application  of  the  resultant 
and  equilibrant  divides  the  line  drawn  perpendicular  to  the  forces, 
inversely  as  the  forces. 

In  Fig.  216,  P  and  Q  are  parallel  forces.    The  resultant  is  a 

force    equal    to   P  +  Q 
acting  downward  at  C. 
The    equilibrant    is    a 
force   equal    to   P  +  Q 
acting    upward    at    C. 
The  point  of  application 
^^^^     C  divides  AB  inversely 
y    as   P  and  Q,  that   is, 
AC'.C  B::Q:P. 


MOTION 

Motion,  velocity.  —  A 

body  is  said  to  be  in 
motion  when  its  position 
is  changing,  and  the 
velocity  of  a  body  is  the 
rate  at  which  its  position 


FIG.  216.  —  The  rod  XY  is  acted  on  by  three 
parallel  forces. 


2 Q2  PHYSICS  OF  THE  HOUSEHOLD 

is  changing.  Velocity  is  expressed  in  various  ways,  feet 
per  second,  centimeters  per  second,  miles  per  hour,  knots  per 
hour,  etc. 

The  space  a  body  travels  in  a  given  time  is  equal  to  its  aver- 
age velocity  multiplied  by  the  time,  that  is : 

space  =  average  velocity  X  time 

Falling  bodies,  acceleration.  —  If  a  body  falls  vertically 
downward,  its  velocity  increases  uniformly.  If  it  is  thrown 
vertically  upward,  its  velocity  decreases  uniformly  until  it 
ceases  to  rise.  The  rate  of  increase  of  velocity  is  called  accel- 
eration; the  rate  of  decrease,  negative  acceleration. 

Experiments  with  falling  bodies.  —  Measure  upward  from 
the  ground  a  vertical  distance  of  16  ft.  Allow  stones  to  drop 
this  distance,  and,  with  a  stop  watch,  measure  the  time  it  takes 
them  to  fall.  It  will  be  found  that  the  stones  fall  the  16  ft.  in 
i  sec. 

Measure  upward  from  the  ground  a  vertical  distance  of 
64  ft.  Allow  stones  to  fall  this  distance  and  find  the  time  of 
fall.  It  will  be  found  that  the  stones  fall  the  64  ft.  in  2  sec. 

If  you  have  a  place  of  sufficient  height,  measure  upward 
from  the  ground  a  vertical  distance  of  144  ft.  Drop  stones  down 
this  distance  and  find  the  time  it  takes  them  to  fall.  It  will 
be  found  that  the  stones  fall  144  ft.  in  3  sec. 

By  these  experiments  it  is  found  that,  on  account  of  the  at- 
traction of  the  earth,  a  body  falls  16  ft.  in  i  sec.,  4  times  as  far 
in  2  sec.,  and  9  times  as  far  in  3  sec.  In  other  words,  the  dis- 
tance the  body  falls  varies  as  the  square  of  the  time. 

Bodies  falling  from  rest.  —  If  we  let  g  equal  the  increase  in 
velocity  or  acceleration  of  a  falling  body,  then  if  the  body  falls 
from  rest  it  has  a  velocity  of  g  ft.  per  second  at  the  end  of  i  sec. ; 
a  velocity  of  2  g  ft.  per  second  at  the  end  of  2  sec. ;  5  g  ft.  per 
second  at  the  end  of  5  sec.,  and  so  on.  In  general  its  final  ve- 
locity v  at  the  end  of  /  seconds  is  gt,  that  is : 


A  FURTHER  STUDY  OF  MECHANICS  293 

The  average  velocity  of  a  falling  body  for  a  given  time  is  the 
average  of  its  initial  velocity  and  its  final  velocity.  If  the  body 
falls  from  rest,  its  initial  velocity  is  o.  In  this  case : 

Average  velocity  =  °  '   £  =  &- 

The  space  any  body  travels  in  a  given  time  is  found  by  mul- 
tiplying its  average  velocity  by  the  time.  In  the  case  of  a  body 
falling  from  rest, 

5  =  average  velocity  X  /  =  j  X  /  =  j2,  that  is,  5  =  %  gt2 

It  has  been  found  that  the  increase  in  velocity  per  second 
(or  acceleration)  of  a  falling  body  is  980  cm.  or  32.16  ft.  (In 
our  calculations  we  will  use  32  ft.  instead  of  32.16  ft.) 

A  body  falling  from  rest  falls  in  i  sec. : 

s  =  %gt*  =  JX32X  i2=  i6ft. 
In  two  seconds  it  falls : 

*  =  \  &*  =  \  X  32  X  4  =  64  ft. 

These  are  the  results  we  obtained  in  our  experiment  above. 
If  a  body  falls  from  rest  for  t  seconds,  its  final  velocity  v  =  gt,- 

then  /  =  -.    If  we  substitute  this  value  of  /  in  the  formula 

g 
s  =  \  gt2  we  obtain  the  formula 


This  gives  the  space  a  body  falls  from  rest  in  terms  of  its 
final  velocity. 

Bodies  given  an  initial  velocity.  —  If  a  body  is  thrown  vertically 
downward  with  a  velocity  of  v  ft.  per  second,  its  velocity  increases 
at  the  rate  of  g  ft.  per  second  and  at  the  end  of  /  sec.,  it  is  v  -f-  gt. 

Its  average  velocity  for  this  time  is  -     ^  "*"  &  '  =  v  +  %  gt. 


294  PHYSICS  OF  THE  HOUSEHOLD 

The  space  this  body  travels  in  t  sec.  is  : 

5  =  average  velocity  X  t  =  (v  +  J  gt)  i 
or  5  =  vt  +  J  gt2 

If  a  body  is  thrown  vertically  upward  with  a  velocity  of 
v  ft.  per  second,  its  velocity  decreases  at  the  rate  of  g  ft.  per 
second,  and  at  the  end  of  /  sec.  it  is  v  —  gt.  Its  average  veloc- 
ity for  this  time  is 

»  +  (»-*)  -  1  ~  §  * 

i  2 

The  space  this  body  travels  in  /  seconds  is  : 

5  =  average  velocity  X  /  =  (v  —  \  gt)  t 
or  s  =  vt  —  J  gt  2 

If  a  body  is  thrown  vertically  upward  with  a  velocity  of  v  ft. 
per  second,  its  velocity  is  decreased  g  ft.  per  second  each  sec- 

ond, therefore  the  time  it  rises  is  -,  that  is  : 

I 

t  =  v- 
g 

If  we  substitute  this  value  for  t  in  the  formula,  s  =  vt  —  %  gt, 
we  obtain  the  formula  : 


This  gives  the  height  to  which  a  body  will  rise  when  its  initial 
velocity  upward  is  v  ft.  per  second. 

It  will  be  noticed  that  this  is  the  same  formula  that  we 
obtained  above,  but  that  5  and  v  stand  for  different  quantities 
in  the  two  formulae. 

Acceleration  in  general.  —  The  letter  a  is  used  to  denote  any 
uniform  acceleration,  the  letter  g  is  used  to  denote  only  the 
acceleration  due  to  the  force  of  gravity.  If  we  substitute  the 
letter  a  for  the  letter  g  in  the  formulae  derived  above,  we  have 
formulae  which  apply  to  any  case  in  which  the  acceleration  is 
uniform. 


A  FURTHER  STUDY  OF  MECHANICS  295 

That  's  J  =  \  at*  gives  the  space  traveled  by  a  body  which 
starts  from  rest  and  travels  for  /  seconds,  the  uniform  accel- 
eration being  a  feet  or  centimeters  per  second. 

The  formula  5  =  vt  +  \  at2  gives  the  space  traveled  in  / 
seconds  by  a  body  having  an  initial  velocity  v  and  an  accelera- 
tion of  a  feet  or  centimeters  per  second. 

The  formula  5  =  vt  —  \  at2  gives  the  space  traveled  in  t 
seconds  by  a  body  -having  an  initial  velocity  of  v  and  a  nega- 
tive acceleration  of  a  feet  or  centimeters  per  second. 

The  formula  s  =  —  gives  :  first,  the  space  a  body  which  starts 

2d 

from  rest  travels  in  acquiring  a  velocity  v,  the  acceleration 
being  a  ;  second,  the  space  a  body  which  has  a  velocity  v  travels 
in  coming  to  rest,  the  negative  acceleration  being  a. 


FORCE 

Newton's  laws  of  motion.  —  The  fundamental  laws  of  motion 
were  stated  by  Sir  Isaac  Newton  over  two  centuries  ago.  They 
are  as  follows  : 

First  Law.  —  A  body  at  rest  remains  at  rest  and  a  body  in  motion 
remains  in  motion  with  constant  speed  in  a  straight  line,  unless 
acted  upon  by  some  external  force. 

Second  Law.  —  Change  of  momentum  is  in  the  direction  of  the 
impressed  force  and  is  proportional  to  it  and  to  the  time  during 
which  it  acts. 

Third  Law.  —  To  every  action  there  is  an  equal  and  opposite 
reaction. 

First  law.  —  The  first  law  defines  a  general  property  of 
matter  ;  namely,  inertia.  Inertia  may  be  defined  as  that  prop- 
erty of  matter  in  virtue  of  which  a  body  at  rest  tends  to  remain  at 
rest  and  a  body  in  motion  tends  to  remain  in  motion  in  a  straight 
line.  All  matter  possesses  the  property  of  inertia.  The 
examples  of  inertia  are  numerous;  three  familiar  examples 
are: 


296  PHYSICS  OF  THE  HOUSEHOLD 

1.  When  a  car  stops  suddenly,  a  person  standing  in  the  car 
tends  to  fall  forward  because  the  inertia  of  his  body  tends  to 
keep  him  going  with  the  same  velocity. 

2.  When  a  car  starts  suddenly,  a  person  standing  tends  to 
fall  backward  because  the  inertia  of  his  body  tends  to  keep  him 
at  rest. 

3.  It  is  difficult  for  a  boy  to  turn  suddenly  when  running 
because  the  inertia  of  his  body  tends  to  keep  him  moving  straight 
ahead. 

Second  law.  —  Change  of  momentum  is  in  the  direction  of 
the  impressed  force  and  is  proportional  to  it  and  to  the  time 
during  which  it  acts.  The  momentum  of  a  body  is  the  quantity 
of  motion  possessed  by  a  body,  and  it  is  defined  as  the  product 
of  the  mass  and  the  velocity  of  the  body : 

Momentum  =  mass  X  velocity 

Thus  a  mass  of  10  Ib.  moving  with  a  velocity  of  20  ft.  per  sec- 
ond has  a  momentum  of  200.  A  mass  of  50  g.  moving  with  a 
velocity  of  100  cm.  per  second  has  a  momentum  of  50  X  100  = 
5000. 

In  comparing  the  momentum  of  one  body  with  that  of  an- 
other care  must  be  taken  to  use  the  same  system  of  measure- 
ment for  both  bodies,  that  is,  pounds  and  feet  or  grams  and 
centimeters. 

The  second  law  gives  us  a  means  of  comparing  forces.  Forces 
are  equal  if  they  produce  equal  changes  of  momentum  in  the 
same  time.  One  force  is  twice  as  great  as  another  if  it  produces 
twice  as  great  a  change  of  momentum  as  the  other  in  a  given 
time. 

Units  of  force.  —  The  common  units  of  force  are  the  pound 
and  the  gram.  These  are  called  the  gravitational  units  of 
force.  A  pound  of  force  is  a  force  equal  to  the  force  the  earth 
exerts  on  a  pound  of  mass.  A  gram  of  force  is  a  force  equal  to 
the  force  the  earth  exerts  on  a  gram  of  mass. 

The  pound  of  force  and  the  gram  of  force  vary  with  the  dis- 


A  FURTHER  STUDY  OF  MECHANICS  297 

tance  of  the  body  from  the  center  of  the  earth.  For  scientific 
purposes  it  is  desirable  to  have  a  constant  unit  of  force. 

In  scientific  work  the  units  of  force  used  are  the  poundal 
and  the  dyne.  A  poundal  is  that  force  which  will  give  a  mass 
of  i  Ib.  an  acceleration  of  i  ft.  per  second,  in  i  sec.  This  unit 
is  used  to  some  extent  in  engineering.  A  dyne  is  that  force 
which  will  give  a  mass  of  i  g.  an  acceleration  of  i  cm.  per  second 
in  i  sec.  This  is  the  unit  used  in  scientific  work. 

From  the  second  law  we  see  that  the  change  of  momentum  is 
proportional  to  the  force  impressed  ;  that  is, 

Force  =  rate  of  change  of  momentum 

The  rate  of  change  of  momentum  is  found  by  multiplying  the 
mass  of  the  body  by  the  change  in  velocity  of  the  body  in  one 
second  or  by  the  acceleration,  therefore 

Force  =  mass  X  acceleration 
or  f  =  ma 

This  is  a  very  important  equation.  It  means  that  the  force  in 
poundals  is  equal  to  the  mass  in  pounds  multiplied  by  the  accel- 
eration in  feet  per  second;  or  that  the  force  in  dynes  is  equal  to 
the  mass  in  grams  multiplied  by  the  acceleration  in  centimeters 
per  second. 

Pound  and  poundal.  —  We  can  find  the  relation  between  the 
pound  and  the  poundal  as  follows.  A  pound  of  force  is  the  force 
which  the  earth  exerts  on  i  pound  of  mass,  and  a  poundal  of 
force  is  the  force  which  gives  i  Ib.  of  mass  an  acceleration  of 
i  ft.  per  second. 

We  know  from  our  experiments  with  falling  bodies  that  when 
a  pound  of  mass  is  allowed  to  fall,  the  i  Ib.  of  force  which  the 
earth  exerts  upon  it  gives  it  an  acceleration  of  32  ft.  per  second. 
Now  from  the  equation  /  =  ma  we  learn  that  the  force  which 
gives  i  Ib.  of  mass  an  acceleration  of  32  ft.  per  second  is  32 
poundals ;  as  follows, 

/  =  ma  poundals 
7=1X32  =  32  poundals 


298  PHYSICS  OF  THE  HOUSEHOLD 

Since  i  Ib.  of  force  and  32  poundals  of  force  each  give  to  i  Ib.  of 
mass  an  acceleration  of  32  ft.  per  second,  we  see  that  i  Ib.of 
force  is  equal  to  32  poundals  of  force.  Since  g  =  32  ft.  per  second, 
the  acceleration  due  to  gravity,  we  can  say  i  Ib.  of  force  is  equal 
to  g  poundals  of  force. 

Gram  and  dyne.  —  We  find  the  relation  between  the  gram  of 
force  and  the  dyne  of  force  in  the  same  way. 

A  gram  of  force  is  the  force  which  the  earth  exerts  upon  i  g. 
of  mass,  and  a  dyne  of  force  is  the  force  which  gives  a  mass  of 
i  g.  an  acceleration  of  i  cm.  per  second. 

When  a  gram  of  mass  is  allowed  to  fall,  the  force  of  i  g.  which 
the  earth  exerts  upon  it  gives  it  an  acceleration  of  980  cm.  per 
second.  From  the  equation  /  =  ma  we  know  that  the  force 
which  gives  i  g.  of  mass  an  acceleration  of  980  cm.  per  second  is 
980  dynes ;  as  follows, 

/  =  ma  dynes 

/  =  i  X  980  =  980  dynes 

Since  i  g.  of  force  and  980  dynes  of  force  each  give  to  i  g.  of 
mass  an  acceleration  of  980  cm.  per  second,  we  see  that  i  g.  of 
force  is  equal  to  980  dynes  of  force.  Since  g  =  980  cm.  per  second, 
the  acceleration  due  to  gravity,  we  can  say  i  gram  of  force  is 
equal  to  g  dynes  of  force. 

Third  law.  —  Action  and  reaction  are  equal  and  opposite. 
This  law  expresses  the  fact  that  there  are  always  two  sides  to 
the  action  of  a  force.  If  a  rope  is  attached  to  a  post  and  a 
man  pulls  on  the  rope  with  a  force  of  10  Ib.,  the  post  resists 
with  a  force  of  10  Ib.  If  a  man  dives  into  the  water  from  a  boat, 
the  man  goes  in  one  direction  with  a  certain  momentum  and 
the  boat  goes  in  the  opposite  direction  with  a  certain  momentum. 
The  third  law  states  that  the  momentum  of  the  boat  backward 
is  equal  to  the  momentum  of  the  man  forward,  etc. 


A  FURTHER  STUDY  OF   MECHANICS  299 

WORK  AND  ENERGY 

Work.  —  On  page  20  we  learned  that  work  is  done  when  a 
force  is  exerted  through  any  distance.  We  learned  there  also 
that  the  units  of  work  in  common  use  are  the  foot  pound  and 
the  gram  centimeter. 

A  foot  pound  of  work  is  done  when  i  Ib.  of  force  is  exerted 
through  a  distance  of  i  ft. 

A  gram  centimeter  of  work  is  done  when  i  g.  of  force  is  exerted 
through  a  distance  of  i  cm. 

Since  the  pound  of  force  and  the  gram  of  force  vary  with  the 
distance  of  a  body  from  the  center  of  the  earth,  the  foot  pound 
and  the  gram  centimeter  are  not  quite  constant.  In  scientific 
work  the  constant  units  of  work,  the  foot  poundal  and  the  erg, 
are  used.  A  foot  poundal  of  work  is  the  work  done  when  a  force 
of  i  poundal  is  exerted  through  a  distance  of  i  ft.  An  erg  of 
work  is  done  when  a  force  of  i  dyne  is  exerted  through  a  distance 
of  i  cm. 

Since  i  pound  is  equal  to  32  poundals,  /  foot  pound  of  work  is 
equal  to  32  foot  poundals  of  work  ;  and  since  i  g.  of  force  is  equal  to  980 
dynes  of  force,  /  gram  centimeter  of  work  is  equal  to  980  ergs  of  work. 

Energy.  —  The  energy  of  a  body  is  defined  as  its  capacity  for 
doing  work.  Inanimate  bodies  possess  energy  only  because 
work  has  been  done  upon  them.  There  are  two  types  of  energy : 
potential  energy  and  kinetic  energy.  The  potential  energy  of  a 
body  is  the  energy  which  it  possess  by  virtue  of  its  position  or 
condition.  For  example,  a  rock  on  top  of  a  cliff  possesses  po- 
tential energy  because  it  can  do  work  in  falling  to  the  base  of 
the  cliff;  a  watch  spring  coiled  up  possesses  potential  energy 
because  it  can  do  work  in  uncoiling.  The  kinetic  energy  of  a 
body  is  the  energy  it  possesses  by  virtue  of  its  mass  and  velocity. 
For  example,  a  bullet  or  pile  driver  in  motion  possess  energy 
because  they  do  work  when  brought  to  rest. 

To  measure  potential  and  kinetic  energy.  —  The  potential 
energy  of  a  body  raised  above  the  earth  is  equal  to  its  mass 
multiplied  by  its  distance  above  the  earth. 


300  PHYSICS  OF  THE  HOUSEHOLD 

PE  =  mh  foot  pounds  or  gram  centimeters 
PE  =  mgh  foot  poundals  or  ergs 

Examples.  —  A  mass  of  10  Ib.  20  ft.  above  the  earth  has  a 
potential  energy  of  10  X  20  =  200  foot  pounds  or  10  X  32 
X  20  =  6400  foot  poundals.  A  mass  of  10  g.  20  cm.  above  the 
earth  has  a  potential  energy  of  10  X  20  =  200  gram  centi- 
meters or  10  X  980  X  20  =  196,000  ergs. 

The  kinetic  energy  of  a  body  is  found  as  follows : 

KE  =  ™^-  foot  pounds  or  gram  centimeters 

2g 

KE  =  —  foot  poundals  or  ergs 

These  equations  are  obtained  as  follows.  When  a  moving  body 
is  brought  to  rest  it  does  work.  For  example,  when  a  moving 
bullet  is  brought  to  rest  by  an  earth  embankment  it  does  work 
in  moving  a  certain  distance  into  the  embankment.  The  work 
it  does  is  equal  to  its  kinetic  energy.  Let  m  =  the  mass  of  the 
moving  body ;  v,  its  velocity ;  s,  the  distance  it  moves  in  being 
brought  to  rest ;  /,  the  force  it  exerts  while  being  brought  to 
rest ;  and  a,  the  decrease  in  its  velocity  per  second  or  its  negative 
acceleration. 

The  kinetic  energy  of  a  moving  body  is  equal  to  the  work  it 
does  in  being  brought  to  rest. 

KE  =  w  =  fs 

but  /  =  ma  and  s  =  — , 

2  a 

therefore  KE=fs  =  maX  —  =  — 

2a        2 

that  is,  KE  =  m-^-  foot  poundals  or  ergs 

2 

This  equation  gives  the  kinetic  energy  in  foot  poundals  or  ergs 
because  in  the  equation  /  =  ma,  f  is  expressed  in  poundals  or 
dynes. 


A  FURTHER   STUDY  OF  MECHANICS  301 

Since  i  Ib.  is  equal  to  g  poundals  and  i  g.  is  equal  to  g  dynes, 
the  kinetic  energy  of  a  moving  body  in  foot  pounds  or  gram 
centimeters  is  found  by  the  equation 

<> 
KE  =  —  foot  pounds  or  gram  centimeters. 

2£ 

Potential  energy  changes  to  kinetic  energy.  —  A  body  at 
rest  above  the  earth  has  potential  energy.  If  the  body  is  al- 
lowed to  fall,  the  potential  energy  changes  continuously  to 
kinetic  energy,  and  at  the  instant  it  strikes  the  ground  its  po- 
tential energy  has  been  changed  entirely  to  an  equal  amount  of 
kinetic  energy  (neglecting  the  friction  of  the  air).  We  can 
illustrate  this  best  by  means  of  an  example. 

A  weight  of  10  Ib.  144  ft.  above  the  earth  has  10  X  144  =  1440 
foot  pounds  of  potential  energy. 

If  this  body  is  allowed  to  fall,  its  velocity  when  it  strikes 

ni2  «.2 

the  ground  is  s  =  —  or  144  =  or  ^  =  64  X  144 

2g  2X32      • 

or  v  =  8  X  12  =  96  ft.  per  second 
Its  kinetic  energy  is 

KE  =  ntf  =  10X96X96  =          foot      unds 

2g  2X32 

That  is,  its  potential  energy  has  been  changed  into  an  equal 
amount  of  kinetic  energy. 

When  a  body  is  shot  upward,  its  kinetic  energy  changes  con- 
tinuously to  potential  energy  and  when  it  reaches  its  greatest 
height  its  potential  energy  is  equal  to  the  kinetic  energy  it  had 
at  the  start. 

The  pendulum.  —  The  pendulum  is  interesting  for  a  number 
of  reasons,  (i)  it  gives  us  an  example  of  potential  energy  chang- 
ing to  kinetic  energy  and  the  reverse ;  (2)  any  given  pendulum 
makes  its  swings  in  equal  times ;  (3)  it  is  the  regulating  device 
of  pendulum  clocks. 

When  the  pendulum  is  in  the  position  A,  Fig.  217,  it  has  po- 
tential energy  equal  to  the  weight  of  the  bob  multiplied  by  the 


302  PHYSICS  OF  THE  HOUSEHOLD 

distance  AH.  When  it  is  in  the  position  N  all  of  this  potential 
energy  has  been  changed  to  kinetic  energy.  When  it  is  in  the 
position  A'  its  kinetic  energy  has  been 
changed  again  to  potential  energy,  etc. 
If  there  were  no  friction,  a  pendulum 
once  started  would  swing  forever  and 
these  changes  would  be  repeated  for 
all  time. 

The  time  of  swing  of  a  given  pendu- 
lum is  approximately  constant.  It  is 
expressed  very  closely  by  the  formula 


H  ar  XT  t  = 

FIG.  217.  —  The  pendulum. 

The  time  /  is  the  time  in  seconds  it 

takes  the  pendulum  to  swing  from  one  side  to  the  other,  for 
example,  from  A'  to  A  or  A  to  A',  /is  the  length  of  the  pendu- 
lum from  the  point  of  support  to  the  center  of  oscillation ;  g  is 
the  force  of  attraction  of  the  earth.  If  /  is  expressed  in  feet, 
g  is  expressed  in  poundals,  g  —  32  poundals.  If  /  is  expressed 
in  centimeters,  g  is  expressed  in  dynes,  g  =  980  dynes. 

EXERCISES 

1.  State  the  law  of  gravitation. 

2.  Define  stable,  unstable,  and  neutral  equilibrium. 

3.  Define  resultant  and  equilibrant. 

4.  State  the  parallelogram  law. 

5.  How  far  will  a  body  fall  in  4  sec. ;  in  6  sec.  ? 

6.  A  body  is  thrown  upwards  with  a  velocity  of  160  ft.  per  second. 
How  many  seconds  will  it  rise? 

7.  How  high  will  the  body  in  (6)  rise  ? 

8.  State  Newton's  laws  of  motion. 

9.  Give  examples  of  inertia. 

10.  Define  poundal,  dyne,  erg,  energy. 

11.  What  is  the  potential  energy  of  a  lo-lb.  body  200  ft.  above  the 
earth? 

12.  What  is  the  kinetic  energy  of  a  lo-lb.  body  moving  with  a  velocity 
of  160  ft.  per  second? 


APPENDIX 

Weights  and  Measures 

Linear  Measure 

12  inches  (in.)  =  i  foot  (ft.)  320  rods     =  i  mile  (mi.) 

3  feet  =  i  yard  (yd.)  1760  yards  =  i  mile 

5.5  yards  =  i  rod  (rd.)  5280  feet     =  i  mile 

16.5  feet  =  i  rod  6  feet     =  i  fathom 


144  square  inches  (sq.  in.) 

9  square  feet 
30^  square  yards 
160  square  rods 
640  acres 

i  square  mile 
36  square  miles 


Square  Measure 


square  foot  (sq.  ft.) 
square  yard  (sq.  yd.) 
square  rod  (sq.  rd.) 
acre  (A.) 
square  mile  (sq.  mi.) 


section 


township 


Cubic  Measure 

1728  cubic  inches  (cu.  in.)  =  i  cubic  foot  (cu.  ft.) 
27  cubic  feet  =  i  cubic  yard  (cu.  yd.) 


Wood  Measure 
=  i  cord  : 
=  i  cord  (cd.) 


16  cubic  feet      =  i  cord  foot  (cd.  ft.) 
128  cubic  feet 
8  cord  feet 

Table  of  Counting 

12  units  =  i  dozen  (doz.)  24  sheets  of  paper         =  i  quire 

12  dozen  =  i  gross  (gro.)  20  quires  or  480  sheets  =  i  ream 

12  gross  =  i  great  gross  (gt.  gro.) 

303 


304  APPENDIX 


Avoirdupois  Weight 

7000  grains  (gr.)  =  i  pound  (Ib.) 
16  ounces  (oz.)  =  i  pound 
100  pounds          =  i  hundredweight  (cwt.) 
2000  pounds          =  i  ton  (T.) 
2240  pounds          =  i  gross  ton 

Troy  Weight 
For  Precious  Metals,  Jewels,  etc. 

24  grains  =  i  pennyweight  (pwt.) 

20  pennyweights  =  i  ounce 
12  ounces  =  i  pound 


437s  grains  =  i  ounce  1  Av 
7000  grains  =  i  pound  j 
480  grains  =  i  ounce  1 
5760  grains  =  i  pound  j 

Apothecaries'  Weight 

20  grains      =  i  scruple  (sc.  or  3 ) 

3  scruples  =  i  dram  (dr.  or   3 )  12  ounces  j  =  j        nd  (lb 

8  drams     =  i  ounce  (oz.  or   3)        S?6o  grains  j 

Apothecaries'  Liquid  Measure 

60  minims          =  i  fluid  dram  (f  3) 
8  fluid  drams  =  i  fluid  ounce  (f  ^ ) 

16  fluid  ounces  =  i  pint  (O.) 
8  pints  =  i  gallon  (cong.) 

Measure  of  Time 

60  seconds  (sec.)  =  i  minute  (min.) 

60  minutes  =  i  hour  (hr.) 

24  hours  =  i  day  (da.) 

7  days  =  i  week  (wk.) 
365  days  or 


1 2  months  (mo.)  J  -  '  yeat  (yr'> 
10  years  =  i  decade 

10  decades  =  i  century 


APPENDIX  305 

Liquid  Measure  (U.  5.) 

4  gills  (gi.)  =  i  pint  (pt.)  231  cu.  in.      =  i  gal. 

2  pints         =  i  quart  (qt.)          31^  gal.  =  L  bbl. 

4  quarts       =  i  gallon  (gal.)  i  liquid  quart  =  5  7. 7  cubic  inches 

Dry  Measure  (U.  S.) 


2  pints  (pt.) 
8  quarts 
4  pecks 
32  quarts 
2150.4  cu.  in. 


quart  (qt.) 
peck  (pk.) 
bushel  (bu.) 
bushel 
bushel 


Liquid  and  Dry  Measure  (British) 

2  pints  (pt.)  =  i  quart  (qt.) 

4  quarts  =  i  gallon  (gal.) 

2  gallons  =  i  peck  (pk.) 

4  pecks  =  i  bushel  (bu.) 

8  bushels  =  i  quarter  (qr.) 

i  quart  =  69.318  cubic  inches 

i  gallon  =  277.274  cubic  inches 

Household  Measures 

i  teaspoon    =  5  c.c.  16  tablespoons  =  i  cup 

3  teaspoons  =  i  tablespoon  2  cups  =  i  pint 

Miscellaneous 

i  (U.  S.)  gallon  of  water  weighs  8.33  Ib. 
i  (British)  gallon  of  water  weighs  10  Ib. 
i  cubic  foot  of  water  weighs  62.3  Ib. 

The  Metric  System 

Measures  of  Length 

10  millimeters  (mm.)  =  centimeter  (cm.) 

10  centimeters  =  decimeter  (dm.) 

10  decimeters  =  meter  (m.) 

10  meters  =  dekameter  (Dm.) 

10  dekameters  =  hektometer  (Hm.) 

10  hektometers  =  kilometer  (Km.) 

10  kilometers  =  myriameter  (Mm.) 
x 


306  APPENDIX 


Measures  of  Surface 


100  sq.  millimeters  (sq.  mm.)  = 

100  sq.  centimeters  = 

100  sq.  decimeters  = 

100  sq.  meters  = 

100  sq.  dekameters  = 

100  sq.  hektometers  = 


sq.  centimeter  (sq.  cm.) 
sq.  decimeter  (sq.  dm.) 
sq.  meter  (sq.  m.) 
sq.  dekameter  (sq.  Dm.) 
sq.  hektometer  (sq.  Hm.) 
sq.  kilometer  (sq.  Km.) 


Measures  of  Volume 

1000  cu.  millimeters  (cu.  mm.)  =  i  cu.  centimeter  (c.c.) 
1000  cu.  centimeters  =  i  cu.  decimeter  (cu.  dm.) 

1000  cu.  decimeters  =  i  cu.  meter  (cu.  m.) 


Measures  of  Capacity 

10  milliliters  (ml.)  =  i  centiliter  (cl.) 

10  centiliters  =  i  deciliter  (dl.) 

10  deciliters  =  i  liter  (1.) 

10  liters  =  i  dekaliter  (Dl.) 

10  dekaliters  =  i  hekoliter  (HI.) 

10  hekoliters  =  i  kiloliter  (Kl.) 


Measures  of  Weight 

10  milligrams  =  i  centigram  10  dekagrams    =  i  hektogram 

10  centigrams  =  i  decigram  10  hektograms  =  i  kilogram 

10  decigrams  =  i  gram  1000  kilograms     =  a  metric  ton 

10  grams  =  i  dekagram 


Commit  to  Memory 

i  meter  (m.)  =  100  centimeters  (cm.) 

i  meter  (m.)  =  1000  millimeters  (mm.) 

i  liter  (1.)  =  1000  cubic  centimeters  (c.c.) 

i  kilogram  (kg.)  =  1000  grams  (g.) 

i  cubic  centimeter  of  water  at  4°  C.  weighs  i  gram 

i  liter  of  water  at  4°  C.  weighs  i  kilogram. 


APPENDIX  307 


Equivalents 

inch  =     2.54  centimeters 

foot  =  30.48  centimeters 

quart  (U.  S.  liquid)  =  .9464  liter 

quart  (U.  S.  dry)  =  i.ioi  liters 

quart  (British)  =  1.1351  liters 

pound  av.  =  .4536  kilogram 

centimeter  =  .3937  inch 

meter  =  39.37  inches 

liter  =  1.051  quarts  (U.  S.  liquid) 

liter  =  .9081  quart  (U.  S.  dry) 

liter  =  .8809  quart  (British) 

i  kilogram  =  2.205  pounds 


INDEX 


Acceleration,  292-295 

Advantage,  4 

Aeroplanes,  75 

Air,  compressed,  57,  62,  64,  70,  73,  75; 
convection  current,  90,  91,  94,  104, 
111-114;  heater,  electric,  190;  weight 
of,  49;  weight  of  hot,  oo 

Air  brake,  76 

Air  chamber,  of  plumbing,  73 ;  of  pump, 
62 ;  of  ram,  64 

Alternating-current  dynamo,  225,  228 

Ammeter,  214 

Ampere,  167,  202,  206 

Anode,  196 

Appliances  in  the  home,  electrical,  163- 
167,  178-180,  182-192,  214-216, 
223-236;  heat,  86-97,  107-114,  133- 
141,  151;  light,  246,  262-268;  me- 
chanical, 2,  6,  7,  9,  15,  31,  62,  63,  66-76 ; 
sound,  279-287 

Arc  light,  193 

Archimedes,  law  of,  40,  56 ;  explanation 
of  law,  42 

Armature,  183,  185,  224 

Artesian  well,  33 

Artificial  ice  machine,  138 

Atmosphere,  pressure  of,  50-55 

Baking  powder,  89 

Balloon,  56,  74 

Barometer,  53 

Batteries,  162-167 

Beats,  286 

Bell,  electric,  178;  circuits,  179 

Bellows,  hydrostatic,  39 

Boiler,  double,  151;  steam,  154 

Boiling,  applications  of,  150;  and  evap- 
oration, 150 

Boiling  point,  148;  variation  of  with 
pressure,  149 

Bolt,  15 

Boyle's  law,  57 

Bread,  mixer,  9,  186;  raising,  88 

British  thermal  unit,  116 


Broiler,  electric,  190 
Button,  push,  179 

Cake  mixer,  186 

Calorie,  116 

Camera,  262 

Can  opener,  6 

Capacity,  heat,  126 

Cathode,  196;    rays,  240 

Cells,    electric,    162-167;    simple,    162; 

storage,  199 
Center  of  gravity,  289 
Centigrade  thermometers,  86 
Chafing  dish,  electric,  189 
Chimney,  draft  in,  92;    location  of,  93 
Circulation,  of  air,  90 ;  of  water,  95 
Clamp,  15 

Clothes,  in;  drying,  146 
Clouds,  147 

Coefficient  of  expansion,  83 
Coffee  mill,  9;    percolator,  electric,  189 
Coherer,  239 
Color,  269;    complementary,  272;    how 

produced,  270 
Commutator,  183,  225 
Compass,  mariner's,  175 
Compressed  air  appliances,   62,  64,  75, 

76 
Conduction,  of  electricity,  168 ;  of  heat, 

102,  106 

Conductivity,  thermal,  106 
Convection,  in  air,  90 ;  in  water,  95 
Cooker,  fireless,  108;  steam,  140 
Cooking,  electric,  appliances,   188;    ex- 
pansion of  gases  in,  88;  utensils,  107, 

118 
Cooling,  artificial,  137 ;   effect  of  ice  and 

ice  water,  122 
Copperplating,  197 
Cost,  of  heat,  153;  of  work,  158 
Coulomb,  206 
Cup  and  cylinder,  41 
Currents,  convection,  90,  95;    electriq 

163  ff. 


309 


3io 


INDEX 


Damper,  in  pipe,  92 ;  in  range,  93 

Daniell  cell,  164 

Density,  43-47 

Dew,  147 ;  point,  148 

Discord,  286 

Dispersion,  270 

Distillation,  140 

Diving  bell,  75 

Draft,  in  grate,  91;    in  range,  93;    in 

stove,  91 
Dry  cell,  166 

Drying,  explanation  of,  146 
Dynamo,    220-288;    alternating-current, 

225;   commercial,  227;  direct-current, 

225;  and  motor,  226 

Echo,  278 

Eggs,  beating,  89;    beater,  186 

Electric,  air  heater,  190;   arc,  193;   bell, 

178;  broiler,  190;  cooking  appliances, 

1 88 ;    coffee  percolator,  189;     chafing 

dish,  189;  heating  appliances,  187-190; 

iron,    187;     luminous   radiator,    189; 

light,   arc,    193;    light,   incandescent, 

191;     motor,    181-186;     oven,    189; 

stove,  189;   telegraphy,  180;   toaster, 

190;  warming  pad,  189 
Electrical,  calculations,  207 ;   sources  of, 

energy,  228;  terms,  202 
Electricity,  how  produced,  162;    in  the 

home,  161  ff. ;    what  is,  161 
Electrolysis,  195 ;  laws  of,  199 
Electromagnet,    175,    177;    applications 

of,  178-186 

Electromotive  force,  167 
Electron,  241 ;  theory  of  matter,  241 
Electroplating,  195-199 
Energy,  299-301 

Engine,  gasoline,  155 ;  steam,  154 
Equilibrant,  290 
Equilibrium,  289 
Erg,  299 
Ether,  104,  250 
Evaporation,  144;    cooling  by,  145;    in 

nature,  147 
Expansion,  80;    applications  of,  84,  88; 

coefficients  of,  83 
Extinguisher,  fire,  70 
Eye,  263 

Fahrenheit  thermometers,  86 
Falling  bodies,  292-295 


Faraday,  199,  220 

Faucet,  15 

Field,  magnetic,  171 

Fire,  extinguisher,  70;  tongs,  7 

Fireless  cooker,  108;  comparing,  120 

Fires,  comparing,  119 

Focal  length,  260 

Fog,  147 

Foot  warmers,  comparing,  120 

Force,  2,  20,  295 ;  arm,  2 ;  units  of,  296 

Forces,    composition   of,  290;    parallel, 

291 ;    parallelogram  law  of,  290 
Fork  and  knife,  7 
Freezing  mixtures,  135 
Friction,  22 
Fruit  press,  6,  15 
Fuels,  152;   comparison  of,  153 
Fulcrum,  2 

Fundamental,  281;  units,  28 
Fusion,  heat  of,  130 

Galvanometer,  213;  D'Arsonval,  213 

Gases,  expansion  of,  88 ;  laws  of,  55-59 

Gas  meter,  73 

Gasoline  engine,  155 

Gold  plating,  198 

Gram,  306 

Grass  cutter,  7 

Grate  shaker,  9 

Gravitation,  law  of,  289 

Gravity,  cell,  165 ;  center  of,  289 

Hail,  147 

Handles,  of  cooking  utensils,  108 

Harmony,  286 

Heat,  appliances  which  control,  106; 
applications  of  latent,  133;  capacity, 
126;  conduction  of,  102;  convection 
.of,  104;  engines,  154;  from  electricity, 
152,  187,  204;  in  the  home,  79; 
latent,  128;  measurement  of,  116; 
mechanical  equivalent  of,  158;  move- 
ment of,  102 ;  nature  of,  99 ;  radiation 
of,  104;  sources  of,  152;  specific,  126; 
turned  to  work,  157;  units  of,  116 

Heating  systems,  hot-air,  95 ;  hot- water, 
96;  steam,  139 

Henry's  law,  59 

Home,  arrangement  of  lighting  fixtures 
in,  246;  electricity  in,  161;  heat  in 
the,  79;  light  in  the,  246;  motor  in 
the,  182 


INDEX 


Horse  power,   157;    of  electric  current, 

210 

Hot-air  heating  system,  95 
Hot-water,  heating  system,  96 ;  tank,  97 
Household  appliances,  electrical,  161  ff. ; 

heat,  79  ff. ;  light,  246  ff. ;  mechanical, 

i  ff. ;  sound,  279  ff. 
Humidity,  148 
Hydraulic  press,  39 
Hydraulic  ram,  64 
Hydrostatic  bellows,  39 
Hydrostatic  paradox,  37 

Ice,  1 23 ;   artificial,  137 ;   and  salt,  136 ; 

latent  heat  of,    128;    machine,    138; 

water,  123 
Images,  by  concave  lens,  262 ;  by  convex 

lens,  260;   in  mirror,  254;   real,  260; 

virtual,  260 

Incandescent  light,  191 
Induced  currents,  210-223;    application 

of,  223-235 
Induction  coil,  232 
Inertia,  295 
Instruments,  musical,   278-287;  optical, 

262-267;   stringed,  280;    wind,  283 
Insulators,  electric,  168;  heat,  106 
Interference,  285 
Iron,  electric,  187 

Jack  screw,  16 

Joule,  206 

Joule's  law,  204,  208 

Key,  telegraph,  180 
Kilogram,  27 

Kilowatt,  202,  206;  hour,  202,  206,  209 
Kinetic  energy,  299 

Kitchen,  motor  in,  185 ;  power  table  in, 
185 

Lamps,  incandescent,  192 ;  arc,  193 

Lantern,  263 

Latent  heat,  of  ice,  128;  of  steam,  130 

Leclanche  cell,  165 

Lemon  squeezer,  6 

Lenses,  259—262 

Lenz  law,  223 

Lever,  i ;  appliances,  2,  6,  7 ;  classes  of, 

5  ;   law  of,  3 
Light,  appliances,  262-268 ;  candle  power 

of,  248;    electric,  192;   in  the  home, 


246 ;    intensity   of,    247 ;     nature   of, 

250;   reflection  of,  253 ;  refraction  of, 

253 

Lighting  fixtures,  arrangement  of,  246 
Lines  of  force,  magnetic,  171 
Liquids,  30 ;  density  of,  45 ;  pressure  in, 

34-40 
Loudness  of  sound,  278 

Machines,  artificial  ice,  138 ;  defined,  14 : 
law  of,  14 

Magdeburg  hemispheres,  50 

Magnet,  artificial,  169;  bar,  171;  elec- 
tro, 175;  horseshoe,  172;  how  made, 
172;  natural,  169;  pole,  169;  theory 
of  molecular,  173. 

Magnetic,  field,  171 ;  field  about  a  cur- 
rent, 176;  induction,  170;  lines  of 
force,  170,  176 

Magnifying  glass,  265 

Mass  and  weight,  28 

Mastery,  4 

Measurement,  24;  common  system  of, 
25»  303;  of  heat,  116  ff. ;  metric 
system  of,  27,  305 

Measuring  instruments,  electrical,  212 

Meat  chopper,  15 ;  grinder,  186 

Mechanical  appliances,  in  the  home,  1-19 

Mechanical  equivalent  of  heat,  158 

Meter,  27 

Metric  system,  305 ;  advantages  of,  27 ; 
history  of,  26 

Microscope,  compound,  266 ;  simple,  265 

Mirror,  254 

Molecular  magnets,  theory  of,  173 

Molecules,  99 

Moment,  2 

Momentum,  296 

Motion,  291 ;  Newton's  laws  of,  295 

Motor,  182 ;  how  current  runs,  183 ;  in 
the  home,  182 

Movement  of  heat,  102  ff. 

Music  and  musical  instruments,  279 

Musical  scale,  279 

Newton,  law  of  gravitation,  289;    laws 

of  motion,  295 
Nickel  plating,  198 
Noise  and  music,  277 
Nonconductors,   of  electricity,    168;   ol 

heat,  106 
Nut  cracker,  6 


312 


INDEX 


Octaves,  281 
Oersted's  discovery,  175 
Ohm,  167,  202,  206 
Ohm's  law,  203,  207 
Opera  glass,  267 
Oven,  electric,  189 
Overtones,  281 

Pad,  electric  wanning,  189 

Paradox  hydrostatic,  37 

Parallelogram  law,  290 

Pascal,  53 ;  law  of,  37,  55 

Pendulum,  301 

Phonograph,  287 

Photometer,  Bunsen's,  249 

Pinsch  gas  system,  77 

Pitch,  277 

Pliers,  2 

Pneumatic    tank,    62;      water    supply 

system,  62 
Pneumatic  tubes,  76 
Polarization,  164 
Power,  horse,  157 
Pressure,  atmosphere,  50-55;    of  gases, 

55 ;  in  liquids,  34-40 ;  in  water  pipes,  40 
Prism,  258 

Pulley,  law  of,  13 ;  systems,  13 
Pump,  air,  65 ;    force,  61 ;    handle,   2 ; 

lift,  61 

Quality,  282 

Radiation,  104,  251 

Radiator,  hot  water,  96;  luminous,  189 

Radioactivity,  244 

Radium,  243 

Rain,  147 

Range,  dampers  in,  93 ;  draft  in,  93 

Rays,  cathode,  240;  light,  252;  X,  242 

Reflection,  253  ;  laws  of,  253 

Refraction,    253,    254;     explanation   of, 

256;  laws  of,  255 
Refrigerators,  109,  133;  comparing,  135; 

temperature  of  air  in,  134 
Relay,  180 
Resistance,  167,  216 
Resonance,  284 
Rheostat,  100 
Right-hand  rules,  176,  178 

Saturated  air,  145 
Scissors,  2 


Screw,   15;    appliances,  15,  18;    advai 

tage  of,  17;   jack,  16;   nail,  15 
Sealer,  15 

Series  connection,  216,  217 
Service  entrance,  215 
Sewing  machine,  motor  driven,  182 
Silver  plating,  198 
Siphon,  71 
Snow,  147 
Sonometer,  280 
Sound,   274;    denned,  276;    velocity  < 

275  ;  waves  of,  276 
Sounder,  telegraph,  180 
Sounding  board,  281 
Specific  heat,  126 
Spectacles,  265 
Spectrum,  269 
Steam,  cooker,  140;   engine,  154;   he* 

ing  effect  of,  123;  heating  system,  13 

latent  heat  of,  130 
Stereoscope,  267 
Still,  domestic,  141 
Storage  cell,  199 
Stove,  draft  in,  92 ;  electric,  188 ;  heati    . 

a  room,  94 
Stringed  instruments,  280 

Tack-lifter,  2 

Telegraph,  key,  180;  relay,  180;  sounder, 

1 80;  wireless,  237 
Telephone,   diagram  of,   234;    receiver 

235;  transmitter,  233 
Telescope,  266 

Thermal  units,  British,  116;  calorie,  ir 
Thermometer,   comparison  of,  87;    g; 

86 ;  liquid,  85,  86 ;  solid,  84 
Thermos  bottle,  109 
Toaster,  electric,  190 
Torricelli,  52 

Transformer,  230;  advantages  of,  231 
Transmitter,  233 
Transparent  substances,  271 
Trap,  72 

Units,  28,  1 1 6,  202 

Vacuum,  sound  in,  275 

Vacuum  cleaners,  66-70;  air  pump  of,  (<e 

Velocity,  291;    of  light,  251;   of  sound, 

275 

Ventilation,  111-113 
Volt,  167,  202,  205 


INDEX 


313 


Voltmeter,  214 

Volume,  of  irregular  solids,  42 

Walls  of  houses,  in 

Washing  machine,  motor  driven,  182 

Water,  electrolysis  of,  195 ;   expansion  of, 

98;    heating  effect  of  boiling,    123; 

hot,  lighter  than  cold,  oo 
Water   supply,    city,    30;     for   country 

homes,  31 ;  pneumatic  tank  system  of, 

63 

Watt,  202,  206,  208;  hour,  202,  206 
Wave,  front,  252;   lengths  of  light,  270; 

length  of  sound,  277 
Wave  theory  of  light,  ^50 
Weight,  2 ;  arm,  3 


Weights  and  measures,  24,  303  ff . 
Wells,  32 ;  artesian,  33 ;  on  hillside,  34 
Wheel  and  axle,  7 ;  appliances,  8 
White  light,  nature  of,  269 
Wind  instruments,  283;    how    sound  is 

started  in,  284 
Wireless  telegraph,  237 
Work,  cost  of,  158;   denned,  20;  law  of, 

21 ;  from  heat,  158;   units  of,  20,  299 
Wringer,  9;  motor  driven,  182 

X-ray,    242;    nature  of,    243;    pictures. 
242 

Yard,  28 ;    stick  as  lever,  i 
Young-Helmholtz  theory,  272 


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few  of  the  Mac  mil  Ian  books  on  kindred  subjects. 


Elementary  Household  Chemistry 

BY 

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In  this  textbook  are  to  be  found  many  new  fea- 
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of  its  subject  matter.  The  princip^s  of  chemistry 
are  developed  and  their  everyday  applications  are 
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of  a  course  given  by  the  author  for  the  last  six 
years  to  students  of  high  school  and  college  age, 
but  of  varying  degrees  of  preliminary  training.  The 
course  has,  therefore,  been  simple  and  the  aim  has 
been  to  introduce  only  such  principles  as  find  imme- 
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Household  Bacteriology 


BY 

ESTELLE    D.   BUCHANAN,   M.S. 
Recently  Assistant  Professor  of  Botany,  Iowa  State  College 

AND 

ROBERT    EARLE    BUCHANAN,   PH.D. 

Professor  of  Bacteriology,  Iowa  State  College,  and  Bacteriologist  of  the 
Iowa  Agricultural  Experiment  Station 

Cloth,  8vo,  xv +  536  pp.,  index,  $2.25 

The  word  Household  is  used  as  an  extension  rather  than  a  limitation  of  the 
title.  In  a  thoroughly  scientihc  manner  the  authors  treat  the  subject-matter 
of  general  as  well  as  of  household  bacteriology  and  include,  therefore,  the 
true  bacteria  as  well  as  the  yeasts,  molds,  and  protozoa.  The  volume  is, 
therefore,  a  general  textbook  of  micro-biology  in  which  special  attention  is 
given  to  those  problems  which  are  of  particular  interest  to  the  student  of 
household  science.  The  main  divisions  cf  the  book  treat  (i)  the  micro- 
organisms themselves,  (2)  fermentations  with  special  reference  to  those 
affecting  foods,  (3)  the  relations  of  bacteria  and  other  micro-organisms  to 
health.  A  fully  illustrated  key  (comprising  37  pages)  to  the  families  and 
genera  of  common  molds  supplements  the  unusually  extended  discussion  of 
the  morphology  and  classification  of  yeasts  and  molds,  and  makes  possible 
the  satisfactory  identification  of  all  forms  ordinarily  encountered  by  the 
student.  The  work  embodies  the  results  of  the  most  recent  researches.  The 
book  is  exceptionally  well  written,  the  different  topics  are  treated  con- 
sistently and  with  a  good  sense  of  proportion.  While  concise  in  statement,  it 
is  thorough  in  method  and  scope.  It  is,  therefore,  well  adapted  for  use  as  a 
text  not  only  f  or  students  of  household  science,  but  also  for  those  to  whom  it  is 
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.  .  .  The  manual  can  be  recommended  as  a  very  good  elementary  bacteri- 
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Textiles 

A  Handbook  for  the  Student  and  the  Consumer 

BY 

MARY   SCHENCK   WOOLMAN,  B.S. 

President  of  the  Women's  Educational  and  Industrial  Union,  Boston,  acting 
of  the  Department  of  Household  Economics,  Simmons  College,  recently 
Professor  of  Domestic  Art  in  Teachers  College 

AND 

ELLEN    BEERS    McGOWAN,   B.S. 

Instructor  in  Household  Arts  in  Teachers  College,  Columbia  University 
Illustrated,  Cloth,  I2mo,  xi  + 428  pp.,  Index,  Bibliography,  $2.00 

This  book  is  the  result  of  twenty  years'  experience  in  teaching  textiles  to 
college  students.  It  is  intended  as  a  textbook  for  college  classes  or  for  study 
clubs  and  as  a  guide  for  the  housekeeper  or  individual  consumer  of  textiles 
and  clothing,  the  teacher,  the  club  woman,  the  saleswoman,  and  as  an  intro- 
ductory survey  of  the  subject  for  the  student  who  contemplates  professional 
work  in  the  textile  industries. 

The  growing  emphasis  upon  textile  study  in  college  departments  of  home 
economics  or  household  arts,  and  the  increasing  use  of  the  textile  industry  as 
teaching  material  in  other  departments  and  other  grades  of  schools,  shows  a 
recognition  of  the  part  that  the  textiles  are  playing  in  the  development  of 
civilization  and  in  our  everyday  life.  Interest  in  the  subject  is  still  further 
accentuated  by  the  movements  now  on  foot  to  regulate  the  social-economic 
conditions  in  the  textile  and  clothing  industries  and  to  secure  standardization 
and  honest  labeling  of  textile  products,  ar>  is  being  done  for  food  products  by 
the  "  pure  food  laws." 

To  meet  the  e;  isting  need  the  authors  have  attempted  to  prepare  a  text 
suitable  for  use  in  college  classes  or  by  the  public,  shorter  and  more  readable 
than  the  technical  handbooks,  yet  sufficiently  thorough  and  comprehensive  to 
give  a  sound  grasp  of  the  subject  as  a  whole  with  so  much  of  the  technology 
as  is  directly  helpful  to  the  consume;  *nd  as  should  be  included  in  general 
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A  Laboratory  Hand-book  for  Dietetics 

By  MARY    SWARTZ    ROSE,    PH.D. 

Assistant  Professor,  Department  of  Nutrition,  Teachers  College,  Columbia  University 

Cloth,  8i'o,  $J.JQ 

Investigations  into  the  quantitative  requirements  of  the  human  body  have  progressed  so 
far  as  to  make  dietetics  to  a  certain  extent  an  exact  science,  and  to  emphasize  the  importance 
of  a  quantitative  study  of  food  materials.  This  little  book  explains  the  problems  involved  in 
the  calculation  of  food  values  and  food  requirements,  and  the  construction  of  dietaries,  and 
furnishes  reference  tables  which  will  minimize  the  labor  involved  in  such  work  without  limiting 
dietary  study  to  a  few  food  materials 

Only  brief  statements  of  the  conditions  affecting  food  requirements  have  been  made,  the 
reader  being  referred  to  general  textbooks  on  the  subject  of  nutrition  for  fuller  information, 
but  such  data  have  been  included  as  seem  most  useful  in  determining  the  amount  of  food  for 
any  normal  individual  under  varying  conditions  of  age  and  activity. 

TABLE  OP  CONTENTS 

PART  I 
Food  Values  and  Food  Requirements. 

THE  COMPOSITION  OF  FOOD  MATERIALS. 
THE  FUNCTIONS  OF  FOOD. 

Food  as  a  Source  of  Energy. 

Food  as  Building  Material 

Food  in  the  Regulation  of  Body  Processes. 
FOOD  REQUIREMENT. 

The  Energy  Requirement  of  Normal  Adults. 

The  Energy  Requirement  of  Children. 

The  Energy  Requirement  of  the  Aged. 

The  Protein  Requirement. 

The  Fat  and  Carbohydrate  Requirement. 

The  Ash  Requirement. 

PART  II 
Problems  in  Dietary  Calculations. 

Studies  in  Weight,  Measure,  and  Cost  of  Some  Common  Food  Materials. 

Relation  between  Percentage  Composition  and  Weight. 

Calulation  of  the  Fuel  Value  of  a  Single  Food  Material. 

Calculation  of  the  Weight  of  a  Standard  or  loo-Calorie  Portion. 

Food  Value  of  a  Combination  of  Food  Materials. 

Distribution  of  Foodstuffs  in  a  Standard  Portion  of  a  Single  Food  Material. 

Calculation  of  a  Standard  Portion  of  a  Combination  of  Food  Materials. 

Analysis  of  a  Recipe. 

Modification  of  Cow's  Milk  to  a  Required  Formula. 

Calculation  of  the  Percentage  Composition  of  a  Food  Mixture. 

The  Calculation  of  a  Complete  Dietary. 

Scoring  of  the  Dietary. 

Reference  Tables. 

Refuse  in  Food  Materials. 

Conversion  Tables  —  Grams  to  Ounces. 

Conversion  Tables  —  Ounces  to  Grams. 

Conversion  Tables  —  Pounds  to  Grams. 

Food  Values  in  Terms  of  Standard  Units  of  Weight. 

Ash  Constituents  in  Percentages  of  the  Edible  Portion. 

Ash  Constituents  in  Standard  or  ico-Calorie  Portions. 

APPENDIX 
The  Equipment  of  a  Dietetics  Laboratory. 


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RETURN  TO  DESK  FROM  WHICH  BORROWED 

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RENEWALS  ONLY— TEL.  NO.  642-3405 
This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


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